feat(frontend): i18n common + nav + auth (Layout, LoginPage, ProtectedRoute)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/LoginPage.tsx

@@ -1,5 +1,6 @@
 import { useState, type FormEvent } from 'react';
 import { Navigate, useLocation, useNavigate } from 'react-router-dom';
+import { useTranslation } from 'react-i18next';
 import { extractApiError } from '@/api/client';
 import { useAuth } from '@/hooks/useAuth';
 import { FormField, TextInput } from '@/components/FormField';
@@ -14,6 +15,7 @@ export function LoginPage(): JSX.Element {
   const location = useLocation();
   const fromPath = (location.state as LocationState | null)?.from;
 
+  const { t } = useTranslation();
   const [username, setUsername] = useState('');
   const [password, setPassword] = useState('');
   const [submitting, setSubmitting] = useState(false);
@@ -34,7 +36,7 @@ export function LoginPage(): JSX.Element {
       await login(username, password);
       navigate(fromPath ?? '/engagements', { replace: true });
     } catch (err) {
-      setError(extractApiError(err, 'Invalid credentials'));
+      setError(extractApiError(err, t('auth.login.invalid')));
     } finally {
       setSubmitting(false);
     }
@@ -48,10 +50,10 @@ export function LoginPage(): JSX.Element {
           <span className="inline-block h-8 w-8 rotate-12 bg-primary" aria-hidden />
           <h1 className="text-[28px] font-medium leading-none">Mimic</h1>
         </div>
-        <p className="text-[16px] text-charcoal">Sign in to access your engagements.</p>
+        <p className="text-[16px] text-charcoal">{t('auth.login.subtitle')}</p>
 
         <form onSubmit={onSubmit} noValidate className="flex flex-col gap-md">
-          <FormField label="Username" htmlFor="login-username" required>
+          <FormField label={t('auth.login.username')} htmlFor="login-username" required>
             <TextInput
               id="login-username"
               name="username"
@@ -62,7 +64,7 @@ export function LoginPage(): JSX.Element {
             />
           </FormField>
 
-          <FormField label="Password" htmlFor="login-password" required>
+          <FormField label={t('auth.login.password')} htmlFor="login-password" required>
             <TextInput
               id="login-password"
               type="password"
@@ -81,7 +83,7 @@ export function LoginPage(): JSX.Element {
           ) : null}
 
           <button type="submit" className="btn-primary" disabled={submitting}>
-            {submitting ? 'Signing in…' : 'Sign in'}
+            {submitting ? t('auth.login.signingIn') : t('auth.login.signIn')}
           </button>
         </form>
       </div>