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/pages/TreeEditorPage.tsx

@@ -102,7 +102,7 @@ export function TreeEditorPage() {
         setLoading(true)
         try {
           const tree = await treesApi.get(id)
-          if (!canEditTree({ author_id: tree.author_id, team_id: tree.team_id })) {
+          if (!canEditTree({ author_id: tree.author_id, account_id: tree.account_id })) {
             navigate('/trees')
             return
           }