test(e2e): robustness — stable locators, exact assertions, no fixed sleep

- ContextSheet: add data-testid="context-sheet". longpress tests targeted the
  open sheet via the `.translate-y-0` animation class (breaks on any animation
  refactor) — now target `[data-testid="context-sheet"][aria-modal="true"]`,
  which is stable and unambiguous vs. the centered LightboxModal (also aria-modal).
- toast-on-failure: the like button was `button.filter(hasText:/\d+/).first()`,
  which could match any digit-bearing button (e.g. the comment count) → use the
  stable aria-label "Gefällt mir".
- config stats: assert the exact user_count (4 = 3 seeded guests + admin) instead
  of `>= 3` — deterministic after the per-test truncate, catches under/overcount.
- offline-network 429 test: replace the fixed 3s waitForTimeout with a
  poll-until-the-retry-count-stabilizes (faster, and a real storm never stabilizes
  → the poll fails, which is the intended outcome).

Note: reviewed the "config restore not in try/finally" finding — it's a non-issue.
The truncate auto-fixture wipes+reseeds the whole config table (and clears the
rate limiter) before every test, so config state cannot leak between tests.

All affected specs verified green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

e2e/specs/05-admin/config.spec.ts

@@ -63,8 +63,10 @@ test.describe('Admin — stats', () => {
     await guest('Stat2');
     await guest('Stat3');
     const stats = await api.getStats(adminToken);
-    // Three guests + the Admin account auto-created on first admin login = 4 users.
-    expect(stats.user_count).toBeGreaterThanOrEqual(3);
+    // Deterministic after the per-test truncate: 3 seeded guests + the Admin account
+    // (recreated by the adminToken fixture's login) = exactly 4. An exact assertion
+    // catches undercount/overcount regressions a `>= 3` lower bound would miss.
+    expect(stats.user_count).toBe(4);
     expect(typeof stats.disk_total_bytes).toBe('number');
   });
 });