feat: unsubscribe, duplicate badges & inbox artwork; fix macOS search/detail

- macOS: fix crash when opening a podcast — two `.searchable` modifiers
  collided in the shared NavigationSplitView toolbar ("duplicate toolbar
  item"). The detail episode search is now inline on macOS; iOS keeps
  `.searchable`.
- macOS: fix the add-podcast search sheet — the source picker was invisible
  (`.principal` does not render in a sheet) and the sheet had no size.
  Replaced with an explicit header (picker + field + close) and a sized
  window. iOS unchanged.
- Subscriptions: right-click / long-press → "Se désabonner" with a
  confirmation, giving macOS the delete affordance it lacked.
- Subscriptions: flag same-title duplicates with an orange "Doublon" badge.
- Nouveautés: show each row's podcast thumbnail via a new optional
  Episode.podcastId carrying the episode→podcast link from storage.

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