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/ProtonStreamTests/UiLogicTests.swift

@@ -20,7 +20,7 @@ private func title(_ key: String, _ episodes: [EpisodeRecord]) -> TitleRecord {
         manualMatch: false, seasons: [SeasonRecord(number: 1, label: "Season 1", episodes: episodes)],
         displayName: key, wideUrl: nil, formatLabel: nil, seasonLabel: nil, studios: [],
         directors: [], tags: [], airing: false, nextEpisode: nil, nextAiringAt: nil,
-        franchise: []
+        franchise: [], continueIndex: nil
     )
 }