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