chore: remove obsolete site.webmanifest and update manifest.ts documentation

- Delete the outdated `site.webmanifest` file to clean up the codebase.
- Enhance documentation in `manifest.ts` to clarify the purpose of the generated web app manifest.

public/site.webmanifest

@@ -1 +0,0 @@
-{"name":"","short_name":"","icons":[{"src":"/android-chrome-192x192.png","sizes":"192x192","type":"image/png"},{"src":"/android-chrome-512x512.png","sizes":"512x512","type":"image/png"}],"theme_color":"#ffffff","background_color":"#ffffff","display":"standalone"}

src/app/manifest.ts

@@ -2,6 +2,8 @@ import { type MetadataRoute } from 'next';
 import { getDefaultMessages } from '@/i18n/messages';
 /**
  * Generates the Web App Manifest for the application
+ * 
+ * generated file name: site.webmanifest
  *
  * ref: https://github.com/amannn/next-intl/blob/main/examples/example-app-router/src/app/manifest.ts
  *