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>

frontend/src/lib/components/ContextSheet.svelte

@@ -108,6 +108,7 @@
 	aria-hidden={!open}
 	inert={!open}
 	tabindex="-1"
+	data-testid="context-sheet"
 >
 	<div class="flex justify-center pt-3 pb-1">
 		<div class="h-1 w-10 rounded-full bg-gray-300 dark:bg-gray-600"></div>