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/page.tsx

@@ -5,6 +5,7 @@ import { useRouter, useSearchParams } from "next/navigation";
 import { useAuth } from "../auth-context";
 import { invitesApi } from "../api";
 import { authFormStyles as styles } from "../styles/auth-form";
+import { LanguageSelector } from "../components/LanguageSelector";
 
 function SignupContent() {
   const searchParams = useSearchParams();
@@ -107,6 +108,9 @@ function SignupContent() {
   if (isCheckingInitialCode) {
     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={{ textAlign: "center", color: "rgba(255,255,255,0.6)" }}>
@@ -122,6 +126,9 @@ function SignupContent() {
   if (!inviteValid) {
     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}>
@@ -189,6 +196,9 @@ function SignupContent() {
   // Step 2: Enter email and password
   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}>