fix(ui): copy GCG to clipboard on Android in-app WebViews

On-device diagnostics from Android Telegram and VK confirmed both expose
no navigator.share AND no navigator.canShare, so the export fell to a Blob
<a download> that those WebViews silently ignore — nothing happened.

pickGcgDelivery is now a 3-way decision: Web Share where available (iOS),
a clipboard copy in an Android in-app WebView (Telegram/VK: no share, dead
download), else a desktop Blob download. shareOrDownloadGcg reports the
outcome so the game shows a "GCG copied" toast; the copy is VKWebAppCopyText
inside VK (which also covers the desktop VK iframe, where navigator.clipboard
is blocked) and navigator.clipboard otherwise.

Unit tests cover the 3-way choice and the copy/failed outcomes; new i18n key
game.gcgCopied (en+ru); docs ARCHITECTURE/FUNCTIONAL(+ru)/UI_DESIGN/TESTING.

ui/src/game/Game.svelte

@@ -21,6 +21,7 @@
   import { alphabetLetters, hasAlphabet } from '../lib/alphabet';
   import { hintsLeft } from '../lib/hints';
   import { shareOrDownloadGcg } from '../lib/share';
+  import { insideVK, vkCopyText } 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';
@@ -837,12 +838,28 @@
 
   async function exportGcg() {
     try {
-      await shareOrDownloadGcg(await gateway.exportGcg(id));
+      const gcg = await gateway.exportGcg(id);
+      const outcome = await shareOrDownloadGcg(gcg, insideTelegram() || insideVK(), copyGcgText);
+      if (outcome === 'copied') showToast(t('game.gcgCopied'), 'info');
+      else if (outcome === 'failed') showToast(t('error.generic'), 'error');
     } catch (e) {
       handleError(e);
     }
   }
 
+  // The GCG export copies to the clipboard in an Android in-app WebView (no Web Share, a dead
+  // <a download>): VKWebAppCopyText inside VK — which also works in the desktop VK iframe, where
+  // navigator.clipboard is blocked — and navigator.clipboard elsewhere.
+  async function copyGcgText(text: string): Promise<boolean> {
+    if (insideVK()) return vkCopyText(text);
+    try {
+      await navigator.clipboard.writeText(text);
+      return true;
+    } catch {
+      return false;
+    }
+  }
+
   // --- move history: open by tapping the score bar, close by tapping or swiping up the board ---
   // The boardwrap surface drives two gestures, selected by `historyOpen`:
   //  - open: the slid board is inert (CSS pointer-events), so the whole board reads as a