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/vite.config.js

@@ -1,11 +1,45 @@
 import { defineConfig } from 'vite'
 import vue from '@vitejs/plugin-vue'
 import tailwindcss from '@tailwindcss/vite'
+import { VitePWA } from 'vite-plugin-pwa'
 
 export default defineConfig({
   plugins: [
     vue(),
     tailwindcss(),
+    VitePWA({
+      registerType: 'autoUpdate',
+      workbox: {
+        globPatterns: ['**/*.{js,css,html,svg,wasm}'],
+        maximumFileSizeToCacheInBytes: 5 * 1024 * 1024,
+        runtimeCaching: [
+          {
+            urlPattern: /^https?:\/\/.*\/api\//,
+            handler: 'NetworkFirst',
+            options: {
+              cacheName: 'api-cache',
+              expiration: { maxEntries: 50, maxAgeSeconds: 300 },
+            },
+          },
+        ],
+      },
+      manifest: {
+        name: 'ROA AUTO - Management Service Auto',
+        short_name: 'ROA AUTO',
+        description: 'Aplicatie de management pentru service-uri auto',
+        theme_color: '#111827',
+        background_color: '#f9fafb',
+        display: 'standalone',
+        orientation: 'portrait-primary',
+        start_url: '/',
+        scope: '/',
+        icons: [
+          { src: '/icon-192.png', sizes: '192x192', type: 'image/png' },
+          { src: '/icon-512.png', sizes: '512x512', type: 'image/png' },
+          { src: '/icon-512.png', sizes: '512x512', type: 'image/png', purpose: 'maskable' },
+        ],
+      },
+    }),
   ],
   server: {
     proxy: {