feat(02-02): frontend auth store, router guard, Login/Register views

- frontend/src/stores/auth.js: useAuthStore with accessToken in memory
  only (never browser storage); login() accepts options.backupCode
- frontend/src/api/client.js: extended with Bearer token injection,
  401 auto-refresh retry, all auth/admin API functions, changePassword
- frontend/src/router/index.js: auth routes added (/login, /register,
  /password-reset, /account, /admin); beforeEach guard redirects
  unauthenticated users to /login with redirect param
- frontend/src/layouts/AuthLayout.vue: centered bare layout for auth pages
- frontend/src/views/auth/LoginView.vue: three-step flow (password, TOTP,
  backup code); "Use a backup code instead" link; UI-SPEC copywriting
- frontend/src/views/auth/RegisterView.vue: registration with
  PasswordStrengthBar; HIBP error display; UI-SPEC copywriting
- frontend/src/components/auth/PasswordStrengthBar.vue: 4-segment bar
- frontend/src/components/ui/AppSpinner.vue: animate-spin SVG spinner
- Stub views: PasswordResetView, NewPasswordView, AccountView, AdminView
- .gitignore: exclude frontend/node_modules, dist, package-lock.json

npm run build exits 0. All acceptance criteria verified.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/AdminView.vue

@@ -0,0 +1,39 @@
+<template>
+  <div class="p-8 max-w-4xl mx-auto">
+    <h2 class="text-2xl font-semibold text-gray-900 mb-4">Admin panel</h2>
+
+    <!-- Sub-navigation -->
+    <div class="flex border-b border-gray-200 mb-6">
+      <button
+        v-for="tab in tabs"
+        :key="tab.id"
+        @click="activeTab = tab.id"
+        class="px-4 py-2 text-sm font-semibold border-b-2 transition-colors"
+        :class="activeTab === tab.id
+          ? 'text-indigo-600 border-indigo-600'
+          : 'text-gray-500 hover:text-gray-700 border-transparent'"
+      >
+        {{ tab.label }}
+      </button>
+    </div>
+
+    <!-- Tab content (stub — wired in Phase 2 admin plan) -->
+    <div class="text-sm text-gray-500">
+      {{ activeTab === 'users' ? 'User management coming soon.' :
+         activeTab === 'quotas' ? 'Quota management coming soon.' :
+         'AI config management coming soon.' }}
+    </div>
+  </div>
+</template>
+
+<script setup>
+import { ref } from 'vue'
+
+const tabs = [
+  { id: 'users', label: 'Users' },
+  { id: 'quotas', label: 'Quotas' },
+  { id: 'ai', label: 'AI Config' },
+]
+
+const activeTab = ref('users')
+</script>