feat: add Web Share Target support

[VCTGomes]

What

Adds Web Share Target API support, allowing the installed PWA to appear
in the native share dialog on Android and other supported platforms.

How it works

• Added share_target to the PWA manifest pointing to / via POST
• Added public/sw-share-target.js which intercepts the POST in the
  Service Worker, extracts the shared files and forwards them to the
  app via postMessage
• Updated index.vue to listen for the postMessage and trigger
  startSendSession when the user selects a peer

Usage

1. Install the PWA on Android
2. Share any file from any app
3. Select LocalSend Web from the share dialog
4. Select the peer to send to

Notes

• Requires the PWA to be installed
• Works with any file type (*/*)
• If multiple peers are available, the user selects which one to send to