Phase 2: Language Context & Selector - Add language dropdown to Header and auth pages

- Create LanguageSelector component with dropdown (shows flag + name)
- Add LanguageSelector to Header (right side, near user email/logout)
- Add LanguageSelector to login, signup, and signup/[code] pages
- Create test-utils.tsx with renderWithProviders helper
- Add vitest.setup.ts to mock localStorage
- Update all test files to use renderWithProviders
- Language selector persists choice in localStorage
- HTML lang attribute updates dynamically based on selected language

All frontend and e2e tests passing.

frontend/vitest.config.ts

@@ -6,6 +6,7 @@ export default defineConfig({
   test: {
     environment: "jsdom",
     include: ["app/**/*.test.{ts,tsx}"],
+    setupFiles: ["./vitest.setup.ts"],
     coverage: {
       provider: "v8",
       reporter: ["text", "html"],