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/03-feed/toast-on-failure.spec.ts

@@ -41,8 +41,9 @@ test.describe('Feed — error toast on user action failures', () => {
     const card = page.locator('article').filter({ hasText: author.displayName }).first();
     await expect(card).toBeVisible({ timeout: 10_000 });
 
-    // Click the like button in the actions row — first visible match inside the card.
-    await card.locator('button').filter({ hasText: /\d+/ }).first().click();
+    // Click the like button by its stable aria-label (the liker hasn't liked yet).
+    // Avoids matching a different digit-bearing button (e.g. the comment count).
+    await card.getByRole('button', { name: 'Gefällt mir' }).click();
 
     // The toast is rendered inside the global Toaster region with aria-live="polite".
     const toast = page.getByTestId('toast').first();