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/app/signup/[code]/page.tsx

@@ -3,6 +3,7 @@
 import { useEffect } from "react";
 import { useRouter, useParams } from "next/navigation";
 import { useAuth } from "../../auth-context";
+import { LanguageSelector } from "../../components/LanguageSelector";
 
 export default function SignupWithCodePage() {
   const params = useParams();
@@ -36,6 +37,9 @@ export default function SignupWithCodePage() {
         fontFamily: "'DM Sans', system-ui, sans-serif",
       }}
     >
+      <div style={{ position: "absolute", top: "1rem", right: "1rem" }}>
+        <LanguageSelector />
+      </div>
       Redirecting...
     </main>
   );