Sync in Settings, render HTML descriptions, fix episodes not loading

- Move the synchronisation configuration into the Réglages screen (SyncView is
  now sections embedded in the Settings form); drop the separate sidebar entry.
- Render feed descriptions as readable text via FeedKit.HTMLText.plain (strip
  tags, decode entities) instead of showing raw HTML. + tests.
- Fix episodes not appearing until a manual pull-to-refresh: the detail used
  `.task` (no id) which didn't re-run when the selected podcast changed in the
  split view. Use `.task(id: podcast.id)` and auto-fetch the feed if the store
  has no episodes yet.

51 package tests green; iOS + macOS build; screens verified in the simulator.

Co-Authored-By: Claude <claude@anthropic.com>