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/store/authStore.ts

@@ -1,11 +1,14 @@
 import { create } from 'zustand'
 import { persist } from 'zustand/middleware'
-import type { User, Token, UserCreate, UserLogin } from '@/types'
+import type { User, Token, UserCreate, UserLogin, Account, SubscriptionDetails } from '@/types'
 import { authApi } from '@/api'
+import { apiClient } from '@/api'
 
 interface AuthState {
   user: User | null
   token: Token | null
+  account: Account | null
+  subscription: SubscriptionDetails | null
   isAuthenticated: boolean
   isLoading: boolean
   error: string | null
@@ -25,6 +28,8 @@ export const useAuthStore = create<AuthState>()(
     (set, get) => ({
       user: null,
       token: null,
+      account: null,
+      subscription: null,
       isAuthenticated: false,
       isLoading: false,
       error: null,
@@ -70,15 +75,30 @@ export const useAuthStore = create<AuthState>()(
         } finally {
           localStorage.removeItem('access_token')
           localStorage.removeItem('refresh_token')
-          set({ user: null, token: null, isAuthenticated: false, error: null })
+          set({ user: null, token: null, account: null, subscription: null, isAuthenticated: false, error: null })
         }
       },
 
       fetchUser: async () => {
         set({ isLoading: true })
         try {
-          const user = await authApi.me()
-          set({ user, isLoading: false })
+          const [userResult, accountResult, subscriptionResult] = await Promise.allSettled([
+            authApi.me(),
+            apiClient.get<Account>('/accounts/me').then(r => r.data),
+            apiClient.get<SubscriptionDetails>('/accounts/me/subscription').then(r => r.data),
+          ])
+
+          const user = userResult.status === 'fulfilled' ? userResult.value : null
+          const account = accountResult.status === 'fulfilled' ? accountResult.value : null
+          const subscription = subscriptionResult.status === 'fulfilled' ? subscriptionResult.value : null
+
+          if (!user) {
+            // User fetch failed — propagate the error
+            const reason = userResult.status === 'rejected' ? userResult.reason : new Error('Failed to fetch user')
+            throw reason
+          }
+
+          set({ user, account, subscription, isLoading: false })
         } catch (error: unknown) {
           const message = error instanceof Error ? error.message : 'Failed to fetch user'
           set({ error: message, isLoading: false })
@@ -95,6 +115,8 @@ export const useAuthStore = create<AuthState>()(
       partialize: (state) => ({
         token: state.token,
         isAuthenticated: state.isAuthenticated,
+        account: state.account,
+        subscription: state.subscription,
       }),
     }
   )