feat(export): platform-native delivery — TG popup+downloadFile, VK viewer, mobile share sheet

Per the on-device review, the last hop is now the most native affordance
per platform, all fed by the same signed URL:

- Telegram: the native showPopup chooser returns (safe here — the whole
  TG chain is bridge calls, which need no user activation) and both
  formats go to the native downloadFile dialog on iOS and Android alike.
- VK: the PNG opens in VK's native photo viewer (VKWebAppShowImages) —
  on screen at once, saved/shared from the viewer's own controls; the
  GCG keeps VKWebAppDownloadFile; either falls back to a plain anchor
  download (the desktop iframe). The gateway now serves /dl/* via
  http.ServeContent (Content-Length + Range/206) — another swing at the
  VK Android DownloadManager hang; if it persists, the next step is the
  clipboard fallback for the VK-Android GCG.
- Mobile browsers: the OS share sheet with the fetched file (the proven
  fetch-then-share pattern) — nothing lands in Downloads first; desktop
  keeps the anchor download.

Legacy Telegram (< 8.0, no downloadFile) keeps the app modal + GCG
clipboard copy and hides the image option.

ui/e2e/export.spec.ts

@@ -58,9 +58,12 @@ test('the chooser downloads the GCG through the same signed-URL route', async ({
   expect((await download).suggestedFilename()).toMatch(/^game-.+\.gcg$/);
 });
 
-test('inside Telegram both formats go through the native downloadFile dialog', async ({ page }) => {
-  // A Telegram stub with downloadFile present (Bot API 8.0): the image option is offered
-  // and the artifact is handed to the native download — never an in-webview anchor.
+test('inside Telegram the chooser is the native popup and delivery the native downloadFile', async ({
+  page,
+}) => {
+  // A Telegram stub with showPopup AND downloadFile (Bot API 8.0): the whole chain stays
+  // in bridge calls — the popup picks the image, the artifact goes to the native download
+  // dialog, never an in-webview anchor. (Activation-safe: no gesture-gated Web APIs.)
   await page.addInitScript(() => {
     Object.assign(window, {
       Telegram: {
@@ -69,6 +72,11 @@ test('inside Telegram both formats go through the native downloadFile dialog', a
           initDataUnsafe: {},
           ready() {},
           expand() {},
+          showPopup(params: { buttons?: { id?: string; type?: string }[] }, cb: (id: string) => void) {
+            const w = window as unknown as { __popup?: unknown };
+            w.__popup = params;
+            setTimeout(() => cb('png'), 0);
+          },
           downloadFile(params: { url: string; file_name: string }) {
             const w = window as unknown as { __downloads?: { url: string; file_name: string }[] };
             (w.__downloads ??= []).push(params);
@@ -84,7 +92,13 @@ test('inside Telegram both formats go through the native downloadFile dialog', a
   await page.locator('.scoreboard').click();
   await page.getByRole('button', { name: 'Export game' }).click();
 
-  await page.getByRole('button', { name: 'Image (PNG)' }).click();
+  // The native popup carried both formats plus cancel, and no in-app modal was mounted.
+  const popup = await page.evaluate(
+    () => (window as unknown as { __popup?: { buttons?: { id?: string; type?: string }[] } }).__popup,
+  );
+  expect(popup?.buttons?.map((b) => b.id ?? b.type)).toEqual(['png', 'gcg', 'cancel']);
+  await expect(page.getByRole('button', { name: 'GCG file' })).toHaveCount(0);
+
   await expect
     .poll(() =>
       page.evaluate(() => (window as unknown as { __downloads?: { url: string; file_name: string }[] }).__downloads),
@@ -98,6 +112,36 @@ test('inside Telegram both formats go through the native downloadFile dialog', a
   expect(dl.file_name).toMatch(/^game-.+\.png$/);
 });
 
+test('a mobile browser gets the OS share sheet with the fetched file', async ({ page }) => {
+  // Web Share with files present (a mobile browser): the artifact is fetched from the
+  // signed URL and handed to navigator.share as a File — the sheet opens directly,
+  // nothing lands in Downloads first.
+  await page.addInitScript(() => {
+    const shared: { name: string; type: string }[] = [];
+    Object.defineProperty(navigator, 'canShare', { value: () => true, configurable: true });
+    Object.defineProperty(navigator, 'share', {
+      value: async (data: { files?: File[] }) => {
+        for (const f of data.files ?? []) shared.push({ name: f.name, type: f.type });
+      },
+      configurable: true,
+    });
+    (window as unknown as { __shared: typeof shared }).__shared = shared;
+  });
+  await routeDl(page);
+  await openFinishedHistory(page);
+  await page.getByRole('button', { name: 'Export game' }).click();
+  await page.getByRole('button', { name: 'Image (PNG)' }).click();
+
+  await expect
+    .poll(() => page.evaluate(() => (window as unknown as { __shared: unknown[] }).__shared))
+    .toHaveLength(1);
+  const shared = await page.evaluate(
+    () => (window as unknown as { __shared: { name: string; type: string }[] }).__shared[0],
+  );
+  expect(shared.name).toMatch(/^game-.+\.png$/);
+  expect(shared.type).toBe('image/png');
+});
+
 test('a legacy Telegram client (no downloadFile) hides the image and copies the GCG', async ({ page }) => {
   await page.addInitScript(() => {
     // Headless engines deny the real clipboard; a permissive stub keeps the legacy

ui/src/game/Game.svelte

@@ -22,12 +22,12 @@
   import { variantNameKey, usesStarBlank, BLANK_STAR } from '../lib/variants';
   import { alphabetLetters, hasAlphabet } from '../lib/alphabet';
   import { hintsLeft } from '../lib/hints';
-  import { downloadUrl, shareOrDownloadGcg } from '../lib/share';
-  import { insideVK, vkCopyText, vkDownloadFile } from '../lib/vk';
+  import { downloadUrl, shareOrDownloadGcg, shareUrlAsFile } from '../lib/share';
+  import { insideVK, vkCopyText, vkDownloadFile, vkShowImages } from '../lib/vk';
   import { getCachedGame, setCachedGame, setCachedDraft, type CachedGame } from '../lib/gamecache';
   import { patchLobbyGame } from '../lib/lobbycache';
   import { applyGameOver, applyMoveDelta, applyOpponentJoined, type DeltaResult } from '../lib/gamedelta';
-  import { insideTelegram, telegramCanDownloadFile, telegramDialogsAvailable, telegramDownloadFile, telegramShowConfirm } from '../lib/telegram';
+  import { insideTelegram, telegramCanDownloadFile, telegramDialogsAvailable, telegramDownloadFile, telegramShowConfirm, telegramShowPopup } from '../lib/telegram';
   import { haptic } from '../lib/haptics';
   import {
     BLANK,
@@ -932,20 +932,37 @@
   }
 
   // The finished-game export offers two formats — the GCG file and the server-rendered PNG
-  // image — behind one 📤 button, always through the app's own modal. Never Telegram's
-  // native popup here: its callback runs with NO user activation, which breaks the
-  // gesture-gated delivery APIs (verified on-device). Both formats travel the SAME route on
-  // every platform: mint a signed relative URL, resolve it against our origin, then hand it
-  // to the platform's native download call — Telegram downloadFile / VKWebAppDownloadFile —
-  // or a plain browser anchor download elsewhere (including the desktop VK iframe, where
-  // the bridge method is unavailable and vkDownloadFile falls through).
+  // image — behind one 📤 button, both minted as one signed relative URL and delivered by
+  // the most platform-native affordance:
+  //   Telegram   native showPopup chooser → native downloadFile dialog. All-native and
+  //              activation-safe: a popup callback carries no user activation, but the
+  //              whole TG chain is bridge calls, never gesture-gated Web APIs.
+  //   VK         our modal chooser (VK has no native one) → the PNG opens in VK's native
+  //              image viewer (VKWebAppShowImages — on screen at once, save/share from the
+  //              viewer's own controls); the GCG goes through VKWebAppDownloadFile; either
+  //              falls back to a plain anchor download when the bridge call fails (the
+  //              desktop iframe is an ordinary browser).
+  //   browsers   our modal → the OS share sheet with the fetched file where files can be
+  //              shared (mobile), else a plain anchor download (desktop).
   let exportOpen = $state(false);
   // The only platform without the URL delivery is a legacy Telegram client predating
   // Bot API 8.0 downloadFile: the GCG falls back to the old clipboard copy there, and
-  // the image option is not offered.
+  // the image option is not offered (and the chooser stays our modal).
   const exportImageAvailable = $derived(!insideTelegram() || telegramCanDownloadFile());
 
-  function onExportClick() {
+  async function onExportClick() {
+    if (insideTelegram() && telegramCanDownloadFile() && telegramDialogsAvailable()) {
+      const choice = await telegramShowPopup({
+        message: t('game.exportChoice'),
+        buttons: [
+          { id: 'png', type: 'default', text: t('game.exportImageOpt') },
+          { id: 'gcg', type: 'default', text: t('game.exportGcgOpt') },
+          { type: 'cancel' },
+        ],
+      });
+      if (choice === 'png' || choice === 'gcg') void exportArtifact(choice);
+      return;
+    }
     exportOpen = true;
   }
 
@@ -963,8 +980,8 @@
   }
 
   // exportArtifact delivers a finished game's artifact by the unified signed-URL route.
-  // The device date locale and the UI-localized non-play labels ride the URL so the
-  // server-rendered image matches what the player would have seen locally.
+  // The device date locale, its IANA time zone and the UI-localized non-play labels ride
+  // the URL so the server-rendered image matches what the player would have seen locally.
   const EXPORT_ACTIONS = ['pass', 'exchange', 'resign', 'timeout'] as const;
   async function exportArtifact(kind: 'png' | 'gcg') {
     if (insideTelegram() && !telegramCanDownloadFile()) {
@@ -978,8 +995,14 @@
       const { path, filename } = await gateway.exportUrl(id, kind, dateLocale, labels, timeZone);
       const url = new URL(path, location.origin).href;
       if (insideTelegram() && telegramDownloadFile(url, filename)) return;
-      if (insideVK() && (await vkDownloadFile(url, filename))) return;
-      downloadUrl(url, filename);
+      if (insideVK()) {
+        if (kind === 'png' && (await vkShowImages(url))) return;
+        if (kind === 'gcg' && (await vkDownloadFile(url, filename))) return;
+        downloadUrl(url, filename);
+        return;
+      }
+      const outcome = await shareUrlAsFile(url, filename, kind === 'png' ? 'image/png' : 'text/plain');
+      if (outcome === 'failed') showToast(t('error.generic'), 'error');
     } catch (e) {
       handleError(e);
     }

ui/src/lib/i18n/en.ts

@@ -311,6 +311,7 @@ export const en = {
   'stats.guestHint': 'Sign in to track your statistics.',
 
   'game.exportGcg': 'Export game',
+  'game.exportChoice': 'Export the finished game as:',
   'game.exportImageOpt': 'Image (PNG)',
   'game.exportGcgOpt': 'GCG file',
   'game.gcgActiveOnly': 'Available once the game is finished.',

ui/src/lib/i18n/ru.ts

@@ -311,6 +311,7 @@ export const ru: Record<MessageKey, string> = {
   'stats.guestHint': 'Войдите, чтобы вести статистику.',
 
   'game.exportGcg': 'Экспорт партии',
+  'game.exportChoice': 'Экспортировать завершённую партию как:',
   'game.exportImageOpt': 'Картинка (PNG)',
   'game.exportGcgOpt': 'Файл GCG',
   'game.gcgActiveOnly': 'Доступно после завершения игры.',

ui/src/lib/share.ts

@@ -97,6 +97,37 @@ export function downloadUrl(url: string, filename: string): void {
   a.remove();
 }
 
+/**
+ * shareUrlAsFile delivers a signed export URL through the OS share sheet where files can
+ * be shared (mobile browsers): it fetches the same-origin bytes, wraps them in a File and
+ * calls navigator.share — the sheet opens directly, nothing lands in Downloads first. The
+ * fetch-then-share chain keeps the click's user activation (the proven GCG pattern).
+ * Where file sharing is absent (desktop) it falls back to the plain anchor download. A
+ * cancelled share is a no-op by the shareOrDownloadGcg invariant.
+ */
+export async function shareUrlAsFile(url: string, filename: string, mime: string): Promise<'shared' | 'downloaded' | 'failed'> {
+  const nav = typeof navigator !== 'undefined' ? navigator : undefined;
+  if (nav && typeof nav.share === 'function' && typeof nav.canShare === 'function') {
+    try {
+      const resp = await fetch(url);
+      if (!resp.ok) return 'failed';
+      const file = new File([await resp.blob()], filename, { type: mime });
+      if (nav.canShare({ files: [file] })) {
+        try {
+          await nav.share({ files: [file], title: filename });
+        } catch {
+          /* cancelled — intentionally a no-op */
+        }
+        return 'shared';
+      }
+    } catch {
+      return 'failed';
+    }
+  }
+  downloadUrl(url, filename);
+  return 'downloaded';
+}
+
 type TextShareNav = Pick<Navigator, 'share'> & { canShare?: Navigator['canShare'] };
 
 /**

ui/src/lib/vk.ts

@@ -155,6 +155,20 @@ export async function vkShare(link: string): Promise<boolean> {
   }
 }
 
+/**
+ * vkShowImages opens VK's native image viewer on url — the image is on screen at once and
+ * the viewer's own controls save/share it, which beats a blind background download for a
+ * picture. Resolves false on any failure or outside VK, so the caller can fall back.
+ */
+export async function vkShowImages(url: string): Promise<boolean> {
+  try {
+    await (await bridge()).send('VKWebAppShowImages', { images: [url] });
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 /**
  * vkDownloadFile downloads url as filename through the VK client (VKWebAppDownloadFile) —
  * the mobile in-app file delivery, where the webview ignores <a download>. Resolves false