feat(web): unlock UX — re-derive DEK on page reload

Solves the core usability problem of zero-knowledge encryption: on page reload
the in-memory DEK is lost, so the user can't decrypt their data even though their
JWT is still valid. Previously they had to close the tab and re-login from scratch.

Changes:
- Persist wrapped_dek/wrapped_dek_iv in the auth store (zustand persist). Safe:
  it's AES-GCM ciphertext, useless without the password KEK — the server already
  stores the same ciphertext. login/register/recover all save the wrapped DEK;
  logout clears it.
- New UnlockPage: minimal password-only form. Re-derives the DEK locally via
  unlockWithPassword (no API call — the JWT is still valid). Falls back to
  deriveAuthAndEncKeys for Phase 1 compat accounts. Links to /login and /recover.
- ProtectedRoute now checks hasEncKey() after isAuthenticated: authenticated but
  no in-memory DEK → redirect to /unlock.
- /unlock route in App.tsx (public, alongside login/register/recover).

Flow: login → browse → reload page → unlock screen → enter password → dashboard
loads with decrypted data. No full re-login needed.

Verified: npm build clean, 20 tests pass.

web/normogen-web/src/components/common/ProtectedRoute.tsx

@@ -1,6 +1,7 @@
 import React from 'react';
 import { Navigate } from 'react-router-dom';
 import { useAuthStore } from '../../store/useStore';
+import { hasEncKey } from '../../crypto';
 
 interface ProtectedRouteProps {
   children: React.ReactNode;
@@ -11,11 +12,11 @@ export const ProtectedRoute: React.FC<ProtectedRouteProps> = ({ children }) => {
 
   if (isLoading) {
     return (
-      <div style={{ 
-        display: 'flex', 
-        justifyContent: 'center', 
-        alignItems: 'center', 
-        height: '100vh' 
+      <div style={{
+        display: 'flex',
+        justifyContent: 'center',
+        alignItems: 'center',
+        height: '100vh'
       }}>
         <p>Loading...</p>
       </div>
@@ -26,5 +27,12 @@ export const ProtectedRoute: React.FC<ProtectedRouteProps> = ({ children }) => {
     return <Navigate to="/login" replace />;
   }
 
+  // Zero-knowledge: the user is authenticated (JWT valid) but the in-memory
+  // encryption key is gone (page reload). Redirect to the unlock screen to
+  // re-derive it without a full re-login.
+  if (!hasEncKey()) {
+    return <Navigate to="/unlock" replace />;
+  }
+
   return <>{children}</>;
 };