feat: add PWA install prompt UI (#35)

- Create usePWAInstall composable for install management
- Add InstallPrompt banner component with auto-show after 3s
- Add App Settings tab in settings page
- Show install button with loading state
- Display installation status and instructions
- Handle dismissal with 7-day cooldown
- Add iOS/Android installation guides
- Show PWA features list
- Display storage usage with visual progress
- Auto-hide prompt after successful install

Features:
- Automatic install prompt after 3 seconds
- Manual install from settings
- Platform-specific instructions
- Smart dismissal tracking
- Storage info visualization

Closes #35

app/pages/settings.vue

@@ -18,6 +18,10 @@
         </div>
       </template>
 
+      <template #app>
+        <SettingsAppSettings />
+      </template>
+
       <template #about>
         <UCard class="mt-4">
           <div class="space-y-4">
@@ -52,6 +56,11 @@ const tabs = [
     label: 'Tags',
     icon: 'i-heroicons-tag'
   },
+  {
+    key: 'app',
+    label: 'App',
+    icon: 'i-heroicons-device-phone-mobile'
+  },
   {
     key: 'account',
     label: 'Account',