Remove manual service worker registration; rely on VitePWA auto-registration

- Deleted src/registerServiceWorker.ts and all related imports
- Cleaned up WebPlatformService and main.web.ts to remove manual SW logic
- Updated VitePWA config for correct dev/prod SW handling
- Fixed missing FontAwesome download icon in PWA prompt
- Updated docs to reflect new PWA registration approach

PWA now works reliably in all web environments with zero manual SW code.

doc/qr-code-implementation-guide.md

@@ -168,7 +168,7 @@ export async function createBuildConfig(mode: string) {
   return defineConfig({
     define: {
       'process.env.VITE_PLATFORM': JSON.stringify(mode),
-      'process.env.VITE_PWA_ENABLED': JSON.stringify(!isNative),
+      // PWA is automatically enabled for web platforms via build configuration
       __IS_MOBILE__: JSON.stringify(isCapacitor),
       __USE_QR_READER__: JSON.stringify(!isCapacitor)
     },