perf(frontend): route-level code splitting, main chunk 538 -> 491 KB

Lighthouse (production, default mobile 4x-CPU profile) scored
Performance 33/100, dominated by one ~536-538 KB entry chunk that
bundled every route's code regardless of which page a visitor landed
on -- the "chunks are larger than 500 kB" Vite warning on every build
this session.

Converted every route except / and the 404 catch-all to React Router
8's native route.lazy (blocks navigation until the chunk is ready --
no separate Suspense fallback needed). AppShell/HomePage/NotFoundPage/
RequireAuth stay eager since they're needed on every load regardless.

Result: main chunk 490.84 KB, no >500kB warning, one small chunk per
route (LoginPage 1.4 KB, ThemesPage 15.5 KB, AccountPage 11 KB, etc.)
loaded only when actually visited.

Fixed a real race the lazy boundary exposed: /login and /register
share an "Почта" field label, and during the chunk swap between them
the old page can stay mounted a beat longer than before -- a test
grabbing the field by label could land on the about-to-unmount one.
Also bumped RTL's asyncUtilTimeout and vitest's testTimeout, since the
route-transition + findBy chain didn't always land inside the old
1000/5000ms defaults under full parallel test-suite load.

Claude-Session: https://claude.ai/code/session_01F1M1Jic1wTSn4igUENynmZ

frontend/vite.config.ts

@@ -35,5 +35,8 @@ export default defineConfig({
     environment: 'jsdom',
     setupFiles: ['./src/test/setup.ts'],
     css: false,
+    // Route pages now load via dynamic import() (src/app/routes.tsx); the default 5000ms
+    // per-test budget can be tight for a route-transition test under a loaded parallel run.
+    testTimeout: 10000,
   },
 })