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/components/Header.tsx

@@ -4,6 +4,7 @@ import { useRouter } from "next/navigation";
 import { useAuth } from "../auth-context";
 import { sharedStyles } from "../styles/shared";
 import constants from "../../../shared/constants.json";
+import { LanguageSelector } from "./LanguageSelector";
 
 const { ADMIN, REGULAR } = constants.roles;
 
@@ -80,6 +81,7 @@ export function Header({ currentPage }: HeaderProps) {
         ))}
       </div>
       <div style={sharedStyles.userInfo}>
+        <LanguageSelector />
         <span style={sharedStyles.userEmail}>{user.email}</span>
         <button onClick={handleLogout} style={sharedStyles.logoutBtn}>
           Sign out