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/hooks/useSubscription.ts

@@ -0,0 +1,45 @@
+import { useAuthStore } from '@/store/authStore'
+
+export function useSubscription() {
+  const subscription = useAuthStore((s) => s.subscription)
+
+  const plan = subscription?.subscription.plan ?? 'free'
+  const limits = subscription?.limits ?? null
+  const usage = subscription?.usage ?? null
+  const isActive = subscription?.subscription.status === 'active' || subscription?.subscription.status === 'trialing'
+
+  const isPaidPlan = plan === 'pro' || plan === 'team'
+
+  const canUseFeature = (feature: 'custom_branding' | 'priority_support'): boolean => {
+    if (!limits) return false
+    return limits[feature]
+  }
+
+  const isAtTreeLimit = (): boolean => {
+    if (!limits || !usage) return false
+    if (limits.max_trees === null) return false // unlimited
+    return usage.tree_count >= limits.max_trees
+  }
+
+  const isAtSessionLimit = (): boolean => {
+    if (!limits || !usage) return false
+    if (limits.max_sessions_per_month === null) return false
+    return usage.session_count_this_month >= limits.max_sessions_per_month
+  }
+
+  const formatLimit = (value: number | null): string => {
+    return value === null ? 'Unlimited' : String(value)
+  }
+
+  return {
+    plan,
+    limits,
+    usage,
+    isActive,
+    isPaidPlan,
+    canUseFeature,
+    isAtTreeLimit,
+    isAtSessionLimit,
+    formatLimit,
+  }
+}