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/UI/Common.swift

@@ -263,9 +263,14 @@ extension View {
     func secureContent() -> some View { modifier(SecureContent()) }
 }
 
-/// Where Play on a title lands: the most recently played part-watched episode,
-/// else the first unwatched one, else the first — the desktop's `next_up`.
-func nextUpIndex(_ playlist: [EpisodeRecord]) -> Int {
+/// Where Play on a title lands: what Continue watching offers
+/// (`TitleRecord.continueIndex`), else the most recently played part-watched
+/// episode, else the first unwatched one, else the first — the desktop's
+/// `next_up`.
+func nextUpIndex(_ playlist: [EpisodeRecord], continueIndex: UInt32? = nil) -> Int {
+    if let index = continueIndex.map(Int.init), playlist.indices.contains(index) {
+        return index
+    }
     if let resumed = playlist.indices.filter({ playlist[$0].resumeAt != nil }).max(by: { playlist[$0].lastPlayed < playlist[$1].lastPlayed }) {
         return resumed
     }