feat(core,app,android,ios): keep a show in Continue watching after an episode ends

Continue watching listed only part-watched episodes, so finishing one -
the most ordinary moment to want the next - dropped the show from the
shelf. Title::continue_with now reads the episode touched last: a
part-watched one is resumed, a finished one offers the episode after it
("Next: S01E04"), and one reset to the start (what leaving the shelf
does) keeps the title off. A few seconds of an episode are passed over.

The phones computed their own shelf from episode records. The bridge now
exports the engine's choice as TitleRecord.continue_index, and Play on a
title lands there on every client.

Undo on the phones re-saved a watch state with a fresh time, which both
reordered the shelf and could not express "never played". A new
restore_watch_state puts the state back as it was read.

ios/ProtonStream/App/AppModel.swift

@@ -516,13 +516,7 @@ final class AppModel {
             do {
                 try await run { engine in
                     for snapshot in snapshots {
-                        try engine.saveWatchState(
-                            shareId: snapshot.shareId,
-                            linkId: snapshot.linkId,
-                            positionSecs: snapshot.state?.positionSecs ?? 0,
-                            durationSecs: snapshot.state?.durationSecs,
-                            watched: snapshot.state?.watched ?? false
-                        )
+                        try engine.restoreWatchState(shareId: snapshot.shareId, linkId: snapshot.linkId, state: snapshot.state)
                     }
                 }
             } catch {