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/fixtures.ts

@@ -0,0 +1,43 @@
+import { test as base, expect } from '@playwright/test';
+
+export { expect };
+export type { Page } from '@playwright/test';
+
+/**
+ * Shared Playwright `test` for the E2E suite.
+ *
+ * Adds an auto-use fallback for the `/api` surface: any request a spec
+ * forgot to mock is fulfilled here with a fast 503 instead of falling
+ * through to the SvelteKit dev proxy. That proxy targets a backend that
+ * isn't running under E2E (`BACKEND_URL` in `.env`), so an unmocked call
+ * otherwise incurs a proxy round-trip + Vite error logging — which, under
+ * 8 parallel workers at cold start, piles up and flakes the first tests.
+ *
+ * Because this route is registered during fixture setup (before the test
+ * body runs), any `page.route(...)` a spec registers later takes
+ * precedence — only genuinely-unmocked calls land here. The 503 also
+ * surfaces mock gaps loudly (a clear warning + a fast, attributable
+ * failure) instead of a 30s "element never appeared" timeout.
+ */
+export const test = base.extend<{ apiFallback: void }>({
+    apiFallback: [
+        async ({ page }, use) => {
+            // Scope to `/api/v1/**` (the real backend surface). A broader
+            // `**/api/**` would also swallow Vite's own dev module requests
+            // under `/src/lib/api/*.ts` and break the app bundle.
+            await page.route('**/api/v1/**', (route) => {
+                const { pathname } = new URL(route.request().url());
+                console.warn(`[e2e] unmocked API call: ${route.request().method()} ${pathname}`);
+                return route.fulfill({
+                    status: 503,
+                    contentType: 'application/json',
+                    body: JSON.stringify({
+                        error: { code: 'e2e_unmocked', message: `unmocked in test: ${pathname}` }
+                    })
+                });
+            });
+            await use();
+        },
+        { auto: true }
+    ]
+});