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/components/layout/ProtectedRoute.tsx

@@ -27,7 +27,7 @@ export function ProtectedRoute({ requiredRole, children }: ProtectedRouteProps)
   if (requiredRole) {
     const ROLE_HIERARCHY: Record<EffectiveRole, number> = {
       super_admin: 4,
-      team_admin: 3,
+      owner: 3,
       engineer: 2,
       viewer: 1,
     }