feat(02-06): Account tab in SettingsView + QR code in TotpEnrollment (GAPs 3, 5)

- frontend/package.json: add qrcode@1.5.4 to runtime dependencies
- TotpEnrollment.vue: import QRCode; generate data URL in startSetup(); render img tag
- SettingsAccountTab.vue: new component with all AccountView content (2FA, password, sessions)
- SettingsView.vue: add Account tab rendering SettingsAccountTab; import SettingsAccountTab

frontend/src/views/SettingsView.vue

@@ -48,6 +48,9 @@
     <!-- Tab: AI Configuration -->
     <SettingsAiTab v-if="activeTab === 'ai'" />
 
+    <!-- Tab: Account -->
+    <SettingsAccountTab v-if="activeTab === 'account'" />
+
     <!-- Tab: Cloud Storage -->
     <div v-if="activeTab === 'cloud'">
       <!-- OAuth error banner (persistent until dismissed) -->
@@ -86,6 +89,7 @@ import { useRouter } from 'vue-router'
 import SettingsPreferencesTab from '../components/settings/SettingsPreferencesTab.vue'
 import SettingsAiTab from '../components/settings/SettingsAiTab.vue'
 import SettingsCloudTab from '../components/settings/SettingsCloudTab.vue'
+import SettingsAccountTab from '../components/settings/SettingsAccountTab.vue'
 
 const router = useRouter()
 
@@ -93,6 +97,7 @@ const tabs = [
   { id: 'preferences', label: 'Preferences' },
   { id: 'ai', label: 'AI Configuration' },
   { id: 'cloud', label: 'Cloud Storage' },
+  { id: 'account', label: 'Account' },
 ]
 
 const activeTab = ref('preferences')