fix: CustomSelect for backup schedule dropdowns, increase PWA cache limit

- Replace native <select> with CustomSelect for hour and day-of-month
  pickers in backup schedule settings (consistent UI)
- Increase PWA workbox cache size limit to 5MB

client/vite.config.js

@@ -8,6 +8,7 @@ export default defineConfig({
     VitePWA({
       registerType: 'autoUpdate',
       workbox: {
+        maximumFileSizeToCacheInBytes: 5 * 1024 * 1024,
         globPatterns: ['**/*.{js,css,html,svg,png,woff,woff2,ttf}'],
         navigateFallback: 'index.html',
         navigateFallbackDenylist: [/^\/api/, /^\/uploads/],