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/login/page.tsx

@@ -4,6 +4,7 @@ import { useState } from "react";
 import { useRouter } from "next/navigation";
 import { useAuth } from "../auth-context";
 import { authFormStyles as styles } from "../styles/auth-form";
+import { LanguageSelector } from "../components/LanguageSelector";
 
 export default function LoginPage() {
   const [email, setEmail] = useState("");
@@ -30,6 +31,9 @@ export default function LoginPage() {
 
   return (
     <main style={styles.main}>
+      <div style={{ position: "absolute", top: "1rem", right: "1rem" }}>
+        <LanguageSelector />
+      </div>
       <div style={styles.container}>
         <div style={styles.card}>
           <div style={styles.header}>