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/package-lock.json

@@ -8,6 +8,7 @@
       "name": "frontend",
       "version": "0.0.0",
       "dependencies": {
+        "@stripe/stripe-js": "^8.7.0",
         "axios": "^1.13.4",
         "class-variance-authority": "^0.7.1",
         "clsx": "^2.1.1",
@@ -1430,6 +1431,15 @@
         "win32"
       ]
     },
+    "node_modules/@stripe/stripe-js": {
+      "version": "8.7.0",
+      "resolved": "https://registry.npmjs.org/@stripe/stripe-js/-/stripe-js-8.7.0.tgz",
+      "integrity": "sha512-tNUerSstwNC1KuHgX4CASGO0Md3CB26IJzSXmVlSuFvhsBP4ZaEPpY4jxWOn9tfdDscuVT4Kqb8cZ2o9nLCgRQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=12.16"
+      }
+    },
     "node_modules/@types/babel__core": {
       "version": "7.20.5",
       "resolved": "https://registry.npmjs.org/@types/babel__core/-/babel__core-7.20.5.tgz",