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/TreeLibraryPage.tsx

@@ -137,6 +137,7 @@ export function TreeLibraryPage() {
     try {
       await treesApi.delete(treeToDelete.id)
       setTrees(trees.filter((t) => t.id !== treeToDelete.id))
+      window.dispatchEvent(new Event('folder-changed'))
     } catch (err) {
       console.error('Failed to delete tree:', err)
       setError('Failed to delete tree')
@@ -352,7 +353,7 @@ export function TreeLibraryPage() {
                     </span>
                     <div className="flex items-center gap-2">
                       <AddToFolderMenu treeId={tree.id} onFolderCreated={handleCreateFolder} />
-                      {canEditTree({ author_id: tree.author_id, team_id: tree.team_id }) && (
+                      {canEditTree({ author_id: tree.author_id, account_id: tree.account_id }) && (
                         <Link
                           to={`/trees/${tree.id}/edit`}
                           className={cn(