feat: update frontend for account-based subscriptions

Replace all team_id/team_admin references with account_id/owner across
types, store, hooks, API clients, components, and pages. Add new
AccountSettingsPage, UpgradePrompt, CheckoutButton, useSubscription
hook, and accounts API client. AuthStore now parallel-fetches account
and subscription data alongside user profile.

Also fix folder sidebar not refreshing after tree deletion by
dispatching the folder-changed event in handleDeleteTree.

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

frontend/src/router.tsx

@@ -10,6 +10,7 @@ import {
   SessionHistoryPage,
   SessionDetailPage,
   SettingsPage,
+  AccountSettingsPage,
 } from '@/pages'
 
 export const router = createBrowserRouter([
@@ -64,6 +65,10 @@ export const router = createBrowserRouter([
         path: 'settings',
         element: <SettingsPage />,
       },
+      {
+        path: 'account',
+        element: <AccountSettingsPage />,
+      },
     ],
   },
 ])