feat: add optional TOTP MFA for email/password accounts

- Add MFA enrollment UI in new Settings page with QR code and backup secret
- Add TOTP challenge step to login flow for enrolled users
- Check AAL after login and show TOTP input when aal2 required
- Add disable MFA option with TOTP re-verification
- Only show MFA options for email/password users (not OAuth)
- Add Settings link to user dropdown menu

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

frontend/src/App.tsx

@@ -12,6 +12,7 @@ import {
   NewRun,
   RunList,
   RunEncounters,
+  Settings,
   Signup,
   Stats,
 } from './pages'
@@ -42,6 +43,7 @@ function App() {
         <Route path="genlockes/new" element={<ProtectedRoute><NewGenlocke /></ProtectedRoute>} />
         <Route path="genlockes/:genlockeId" element={<GenlockeDetail />} />
         <Route path="stats" element={<Stats />} />
+        <Route path="settings" element={<ProtectedRoute><Settings /></ProtectedRoute>} />
         <Route
           path="runs/:runId/encounters"
           element={<Navigate to=".." relative="path" replace />}