feat(ui): diagnostic screen on /telegram/ instead of the landing bounce

A Mini App launch on /telegram/ without sign-in data (empty initData) used to
location.replace('/') — bouncing the visitor to the marketing landing. That
destroyed all diagnosability and, on some Android clients that reach the entry
with no initData, simply looked like the app refusing to open.

Replace the bounce with a compact, privacy-safe launch-error screen
(screens/TelegramLaunchError.svelte) that renders a one-screenshot diagnostic
snapshot captured at the moment of failure: SDK/WebApp presence, Telegram
platform/version, whether initData is empty, whether the URL fragment carried
tgWebAppData, the initData field NAMES present/missing (never the signed
values, never an IP), and OS/mobile/browser via User-Agent Client Hints plus
the full User-Agent. A Share button delivers it through the OS share sheet
(clipboard copy on desktop, reusing the GCG no-webview-strand guard); Retry
re-checks in place to recover a late initData without a reload (which would
discard the launch fragment).

This is the instrument to root-cause the Android empty-initData failure, which
is not reproducible in Playwright.

Tests: collectTelegramDiag + shareText/pickTextShare unit tests; the /telegram/
e2e now asserts the diagnostic screen, not a redirect. Docs: ARCHITECTURE.md +
UI_DESIGN.md updated.

ui/src/lib/share.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from 'vitest';
-import { pickGcgDelivery, shareOrDownloadGcg } from './share';
+import { pickGcgDelivery, pickTextShare, shareOrDownloadGcg, shareText } from './share';
 import type { GcgExport } from './model';
 
 const file = {} as File;
@@ -60,3 +60,42 @@ describe('shareOrDownloadGcg', () => {
     expect(anchor.click).toHaveBeenCalledOnce();
   });
 });
+
+describe('pickTextShare', () => {
+  it('shares when Web Share is available', () => {
+    expect(pickTextShare({ share: async () => {}, canShare: () => true })).toBe('share');
+  });
+
+  it('shares when share exists without canShare (text needs no file capability check)', () => {
+    expect(pickTextShare({ share: async () => {} })).toBe('share');
+  });
+
+  it('copies when there is no Web Share (desktop)', () => {
+    expect(pickTextShare(undefined)).toBe('copy');
+    expect(pickTextShare({} as never)).toBe('copy');
+  });
+});
+
+describe('shareText', () => {
+  afterEach(() => vi.unstubAllGlobals());
+
+  it('uses the OS share sheet when available', async () => {
+    const share = vi.fn().mockResolvedValue(undefined);
+    vi.stubGlobal('navigator', { share, canShare: () => true });
+    expect(await shareText('diag', 'title')).toBe('shared');
+    expect(share).toHaveBeenCalledWith({ title: 'title', text: 'diag' });
+  });
+
+  it('copies to the clipboard when Web Share is absent (desktop)', async () => {
+    const writeText = vi.fn().mockResolvedValue(undefined);
+    vi.stubGlobal('navigator', { clipboard: { writeText } });
+    expect(await shareText('diag', 'title')).toBe('copied');
+    expect(writeText).toHaveBeenCalledWith('diag');
+  });
+
+  it('reports failure without a fallback when the share is cancelled', async () => {
+    const share = vi.fn().mockRejectedValue(new DOMException('cancelled', 'AbortError'));
+    vi.stubGlobal('navigator', { share, canShare: () => true });
+    expect(await shareText('diag', 'title')).toBe('failed');
+  });
+});