Lists: watch state and self-service watch/unwatch #68

Gap

The app manages watchers as an owner (invite/list/remove) and shows "Shared with me" via GET /api/lists/watching, but never reads its own watch state for a given list.

GET /api/lists/{id}/watchers/me → live-verified 200 {"watching":false,"role":null}.

Acceptance criteria

• Service method + model.
• A list detail shows whether the viewer is watching it and at what role.
• The role drives which affordances appear (read-only vs edit vs admin).
• Owner-managed watcher UI and this viewer-side state do not contradict each other.