feat(frontend): PWA + Backup/Restore + Upgrade Prompts + Settings

- PWA: vite-plugin-pwa with autoUpdate service worker, manifest, offline
  caching for assets + NetworkFirst for API calls
- PWA icons: 192x192 and 512x512 placeholder PNGs + favicon.svg
- index.html: theme-color, apple-touch-icon, description meta tags
- UpgradeBanner component: trial expiry warning with upgrade CTA
- SettingsView: complete settings page with:
  - Plan info display
  - Tenant profile form (firma, CUI, reg com, adresa, IBAN, banca)
  - Backup export (JSON with all tenant data from wa-sqlite)
  - Restore import (JSON file with validation and INSERT OR REPLACE)
  - User management with invite form (email + rol)
  - Logout button
- useOffline composable: shared reactive online/offline state
- DashboardView: added UpgradeBanner at top

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

frontend/public/favicon.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
+  <rect width="100" height="100" rx="20" fill="#111827"/>
+  <text x="50" y="38" font-family="Arial,sans-serif" font-size="22" font-weight="bold" fill="#fff" text-anchor="middle">ROA</text>
+  <text x="50" y="68" font-family="Arial,sans-serif" font-size="22" font-weight="bold" fill="#3b82f6" text-anchor="middle">AUTO</text>
+</svg>