FDM v2.8.7

feat(2.8.7): magnet OS handler + self-healing torrent engine

Magnet deep links
- Registered FDM as the OS handler for magnet: links via tauri-plugin-deep-link
  (scheme in tauri.conf.json, deep-link:default capability). Clicking a magnet in
  a browser opens FDM and queues the torrent.
- Handles every entry path: macOS Apple-event (running + cold launch), Windows/
  Linux argv, and single-instance forwarding. Rust buffers links (PendingMagnets)
  + emits open-magnet; the frontend drains any launch links via take_pending_magnets
  and listens live, queuing into the default download folder.

Torrent engine self-heals
- rqbit's cached API base was reused with no liveness check, so once the sidecar
  died every add/stats failed with "error sending request" and the graceful retry
  looped forever. ensure() now probes the cached engine and respawns a fresh rqbit
  if it isn't answering.

Note: the magnet scheme registers with the OS only in an installed/bundled build
(macOS Info.plist CFBundleURLTypes); dev doesn't register it, Windows is CI-gated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>