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/tree-editor/TreeMetadataForm.tsx

@@ -119,7 +119,7 @@ export function TreeMetadataForm() {
             {categories.map((cat) => (
               <option key={cat.id} value={cat.id}>
                 {cat.name}
-                {cat.team_id ? ' (Team)' : ''}
+                {cat.account_id ? ' (Account)' : ''}
               </option>
             ))}
             <option value="__custom__">+ Add custom category</option>