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/common/UpgradePrompt.tsx

@@ -0,0 +1,32 @@
+import { cn } from '@/lib/utils'
+import { useSubscription } from '@/hooks/useSubscription'
+
+interface UpgradePromptProps {
+  feature: string  // e.g., "create more trees", "start more sessions"
+  className?: string
+}
+
+export function UpgradePrompt({ feature, className }: UpgradePromptProps) {
+  const { plan } = useSubscription()
+
+  return (
+    <div className={cn(
+      'rounded-lg border border-yellow-500/30 bg-yellow-500/10 p-4',
+      className
+    )}>
+      <h3 className="font-semibold text-foreground">Plan Limit Reached</h3>
+      <p className="mt-1 text-sm text-muted-foreground">
+        Your {plan} plan doesn't allow you to {feature}. Upgrade your plan to continue.
+      </p>
+      <button
+        className={cn(
+          'mt-3 rounded-md bg-primary px-4 py-2 text-sm font-medium text-primary-foreground',
+          'hover:bg-primary/90'
+        )}
+        onClick={() => window.location.href = '/account'}
+      >
+        View Plans
+      </button>
+    </div>
+  )
+}