test(e2e): kill cold-start flakiness with an /api/v1 fallback + one retry

A full parallel run intermittently failed the first few tests: unmocked
/api/v1 calls fell through to the dev proxy, whose backend isn't running
under E2E, so each incurred a slow ECONNREFUSED round-trip + Vite error
logging that piled up across 8 workers at cold start.

- Add e2e/fixtures.ts: a shared `test` whose auto-use fixture installs an
  `**/api/v1/**` fallback (fast 503, logged) for any endpoint a spec
  didn't mock. Registered before the test body, so per-test routes still
  win; only genuine gaps land here. Turns silent mock-gap hangs into
  instant, attributable failures — and cuts the full run ~96s → ~25s.
- Point all 22 specs at ./fixtures instead of @playwright/test.
- retries: 1 in the config as a safety net for the residual Vite
  cold-compile timing (re-runs on the now-warm server).

Two consecutive full runs: 110 passed, 0 flaky, no retries consumed.

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

frontend/e2e/reader-chapter-select.spec.ts

@@ -1,4 +1,4 @@
-import { test, expect, type Page } from '@playwright/test';
+import { test, expect, type Page } from './fixtures';
 
 const mangaId = '33333333-3333-3333-3333-333333333333';
 const chapter1Id = 'c1111111-3333-3333-3333-333333333333';