fix(continue-watching): clear the resume position when an item is marked watched

Marking a movie or episode watched left it sitting in Continue Watching with a
checkmark, and the only way to shift it was to play it and skip to the end.

Continue Watching membership on a MediaBrowser server is derived from
UserData.PlaybackPositionTicks alone; Played is never consulted. Marking played
normally zeroes that position as a side effect, so the row usually disappears
and nothing ever checked that it had. When something writes a position back
afterwards the item is left played *and* resumable, which the resume route
happily keeps returning forever. markWatched now reads the UserItemDataDto the
mark already returns and clears the bookmark itself when the server left one
behind, so the postcondition holds however the item got into that state. The
follow-up write costs a request only when the invariant is actually broken.

The writer putting items there is our own offline queue. insertWatchAction
already drops queued progress for an item when the mark is itself queued, but
the online mark writes straight to the server and queues nothing, so a progress
row recorded earlier survived and replayed afterwards — pending actions go out
oldest first — restoring the very position the mark had cleared. The sync
service now listens for watch-state events and discards queued progress for
that item as the mark lands. Progress recorded after a mark is a rewatch and is
queued later, so it is untouched. Plex never showed this because it forwards the
recorded-at timestamp and lets the server discard a stale replay; the
MediaBrowser stop report has nowhere to put one.

Continue Watching also drops the row locally now instead of waiting a round trip
for the refetch to confirm it, matching what removal events already did, and
marking a season or show takes its on-deck episode with it.

Watched items are deliberately still not filtered out of the shelf: Jellyfin
keeps Played set when new progress arrives, so a rewatch in progress is
indistinguishable from a stuck row, and filtering would hide it.

close #1812

lib/services/media_browser_paths.dart

@@ -34,6 +34,18 @@ class MediaBrowserPaths {
   String playedItem(String itemId) =>
       dialect.requiresUserScopedItemRoutes ? '$_user/PlayedItems/${_id(itemId)}' : '/UserPlayedItems/${_id(itemId)}';
 
+  /// Per-user playback-state write. `POST` with `{"PlaybackPositionTicks": 0}`
+  /// clears the resume bookmark while leaving `Played` untouched (verified on
+  /// Jellyfin 10.11.10 for both spellings).
+  ///
+  /// Continue Watching membership on this API is derived purely from
+  /// `UserData.PlaybackPositionTicks > 0` — `Played` is not consulted — so this
+  /// is the only route that can guarantee a finished item stops being
+  /// resumable. See [MediaServerClient.markWatched].
+  String userItemData(String itemId) => dialect.requiresUserScopedItemRoutes
+      ? '$_user/Items/${_id(itemId)}/UserData'
+      : '/UserItems/${_id(itemId)}/UserData';
+
   /// Favourite flag write route (`POST` to add, `DELETE` to remove).
   String favoriteItem(String itemId) => dialect.requiresUserScopedItemRoutes
       ? '$_user/FavoriteItems/${_id(itemId)}'