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/subscription/CheckoutButton.tsx

@@ -0,0 +1,24 @@
+import { cn } from '@/lib/utils'
+
+interface CheckoutButtonProps {
+  plan: 'pro' | 'team'
+  className?: string
+}
+
+export function CheckoutButton({ plan, className }: CheckoutButtonProps) {
+  const planLabels = { pro: 'Pro', team: 'Team' }
+
+  return (
+    <button
+      disabled
+      title="Billing coming soon"
+      className={cn(
+        'rounded-md bg-primary px-4 py-2 text-sm font-medium text-primary-foreground',
+        'disabled:opacity-50 disabled:cursor-not-allowed',
+        className
+      )}
+    >
+      Upgrade to {planLabels[plan]} (Coming Soon)
+    </button>
+  )
+}