Stop tracking resume progress for live streams

With "Partially Watched Videos" set to Ask, opening a live stream showed
the resume sheet with a playback timestamp. A live position is relative
to the live edge, not a fixed timeline, so it is meaningless as a
playhead.

Live views are still recorded in history, but carry no resume position:

- WatchEntry gains a persisted isLive flag plus recordLiveWatch(), which
  stores no position and clears state a drifting HLS duration may have
  left behind. progress is always 0 and toVideo() propagates the flag so
  history rows render the LIVE badge.
- from(video:) clamps duration, keeping Piped's -1 live sentinel out of
  storage.
- DataManager routes live saves through recordLiveWatch() and clears the
  flag on the non-live path (before updateProgress, so the auto-finish
  check is not suppressed by the hard-0 live progress), letting an entry
  heal once the stream becomes a VOD.
- PlayerService skips completion saves, always starts at the live edge,
  and drops any startTime passed for a live video.
- The resume prompt, the "Continue at" button label, thumbnail progress
  bars, "X remaining", Continue Watching and tvOS Top Shelf all exclude
  live entries.
- CloudKit syncs isLive with the rest of the watch state: the conflict
  resolver carries it with watchedSeconds/isFinished and the sync engine
  applies it when merging into an existing entry. It is read leniently,
  so the schema version stays at 2 and older clients keep parsing these
  records.

Yattee/Services/TopShelfSnapshotWriter.swift

@@ -54,7 +54,7 @@ enum TopShelfSnapshotWriter {
         guard let dataManager else { return }
         let history = dataManager.watchHistory(limit: 50)
         let items = history
-            .filter { !$0.isFinished && $0.watchedSeconds > 10 }
+            .filter { !$0.isFinished && !$0.isLive && $0.watchedSeconds > 10 }
             .prefix(TopShelfSnapshot.maxItems)
             .compactMap(Self.makeItem(from:))
         TopShelfSnapshot.write(Array(items), section: .continueWatching)