feat: password unlock screen on refresh instead of full logout

On page reload, tokens persist but crypto keys are lost (by design).
Now shows a password-only unlock screen instead of the full login form,
so users don't need to re-enter their email after a refresh.

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

frontend/src/pages/LoginPage.tsx

@@ -1,6 +1,6 @@
 import { useState, FormEvent } from 'react';
-import { Link, useNavigate } from 'react-router-dom';
-import { Shield } from 'lucide-react';
+import { Link, useNavigate, useLocation } from 'react-router-dom';
+import { Shield, Lock } from 'lucide-react';
 import api from '../lib/api';
 import { useAuthStore } from '../lib/store';
 import { deriveKeyFromPassword, decryptMasterKey, decryptPrivateKeyToCryptoKey } from '../crypto';
@@ -8,7 +8,12 @@ import { socket } from '../lib/ws';
 
 export default function LoginPage() {
   const navigate = useNavigate();
-  const { setAuth, setMasterKey, setKeyPair, setUsername } = useAuthStore();
+  const location = useLocation();
+  const { setAuth, setMasterKey, setKeyPair, setUsername, username, email: storedEmail } = useAuthStore();
+
+  // Reauth: tokens still valid but crypto keys lost on refresh
+  const isReauth = location.state?.reauth === true && !!storedEmail;
+
   const [email, setEmail] = useState('');
   const [password, setPassword] = useState('');
   const [totpCode, setTotpCode] = useState('');
@@ -23,7 +28,7 @@ export default function LoginPage() {
 
     try {
       const res = await api.post('/auth/login', {
-        email,
+        email: isReauth ? storedEmail : email,
         password,
         ...(totpCode ? { totpCode } : {}),
       });
@@ -34,18 +39,15 @@ export default function LoginPage() {
         return;
       }
 
-      const { accessToken, refreshToken, userId, username, kdfSalt, encryptedMasterKey, isAdmin } = res.data;
+      const { accessToken, refreshToken, userId, username: uname, kdfSalt, encryptedMasterKey, isAdmin } = res.data;
 
-      // Re-derive master key from password — never leaves the browser
       const derived = await deriveKeyFromPassword(password, kdfSalt);
       const masterKey = await decryptMasterKey(encryptedMasterKey, derived);
 
-      // Set tokens first so the next API call has a Bearer token
-      setAuth({ accessToken, refreshToken, userId, isAdmin });
+      setAuth({ accessToken, refreshToken, userId, isAdmin, email: isReauth ? storedEmail! : email });
       setMasterKey(masterKey);
-      setUsername(username);
+      setUsername(uname);
 
-      // Fetch and decrypt the X25519 private key (stored encrypted on server for cross-device support)
       const keyRes = await api.get('/auth/my-key');
       const privateKey = await decryptPrivateKeyToCryptoKey(keyRes.data.encrypted_private_key, masterKey);
       setKeyPair(keyRes.data.public_key, privateKey);
@@ -63,17 +65,27 @@ export default function LoginPage() {
     <div className="min-h-screen flex items-center justify-center p-4">
       <div className="w-full max-w-sm">
         <div className="flex flex-col items-center mb-8">
-          <Shield className="w-12 h-12 text-accent mb-3" />
+          {isReauth ? (
+            <Lock className="w-12 h-12 text-accent mb-3" />
+          ) : (
+            <Shield className="w-12 h-12 text-accent mb-3" />
+          )}
           <h1 className="text-2xl font-bold">CloudLink</h1>
-          <p className="text-slate-400 text-sm mt-1">End-to-end encrypted</p>
+          <p className="text-slate-400 text-sm mt-1">
+            {isReauth ? `Welcome back, ${username}` : 'End-to-end encrypted'}
+          </p>
         </div>
 
         <form onSubmit={submit} className="bg-surface-1 rounded-xl p-6 space-y-4 border border-slate-700">
-          <h2 className="font-semibold text-lg">Sign in</h2>
+          <h2 className="font-semibold text-lg">
+            {isReauth ? 'Enter your password to unlock' : 'Sign in'}
+          </h2>
 
           {error && <p className="text-red-400 text-sm">{error}</p>}
 
-          {!require2fa ? (
+          {isReauth ? (
+            <Field label="Password" type="password" value={password} onChange={setPassword} autoFocus />
+          ) : !require2fa ? (
             <>
               <Field label="Email" type="email" value={email} onChange={setEmail} />
               <Field label="Password" type="password" value={password} onChange={setPassword} />
@@ -95,21 +107,33 @@ export default function LoginPage() {
           )}
 
           <button type="submit" disabled={loading} className="btn-primary w-full">
-            {loading ? 'Signing in…' : require2fa ? 'Verify' : 'Sign in'}
+            {loading ? (isReauth ? 'Unlocking…' : 'Signing in…') : isReauth ? 'Unlock' : require2fa ? 'Verify' : 'Sign in'}
           </button>
+
+          {isReauth && (
+            <button
+              type="button"
+              onClick={() => { useAuthStore.getState().logout(); navigate('/login'); }}
+              className="w-full text-center text-sm text-slate-500 hover:text-slate-300"
+            >
+              Sign in as a different user
+            </button>
+          )}
         </form>
 
-        <p className="text-center text-sm text-slate-400 mt-4">
-          No account?{' '}
-          <Link to="/register" className="text-accent hover:underline">Register</Link>
-        </p>
+        {!isReauth && (
+          <p className="text-center text-sm text-slate-400 mt-4">
+            No account?{' '}
+            <Link to="/register" className="text-accent hover:underline">Register</Link>
+          </p>
+        )}
       </div>
     </div>
   );
 }
 
-function Field({ label, type, value, onChange }: {
-  label: string; type: string; value: string; onChange: (v: string) => void;
+function Field({ label, type, value, onChange, autoFocus }: {
+  label: string; type: string; value: string; onChange: (v: string) => void; autoFocus?: boolean;
 }) {
   return (
     <div>
@@ -119,6 +143,7 @@ function Field({ label, type, value, onChange }: {
         type={type}
         value={value}
         onChange={(e) => onChange(e.target.value)}
+        autoFocus={autoFocus}
         required
       />
     </div>