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.

android/app/src/test/java/io/narl/protonstream/ui/HistoryTest.kt

@@ -26,6 +26,7 @@ class HistoryTest {
         displayName = key, wideUrl = null, formatLabel = null, seasonLabel = null, studios = emptyList(),
         directors = emptyList(), tags = emptyList(), airing = false, nextEpisode = null, nextAiringAt = null,
         franchise = emptyList(),
+        continueIndex = null,
     )
 
     @Test

android/app/src/test/java/io/narl/protonstream/ui/ScreenshotTest.kt

@@ -264,6 +264,9 @@ class ScreenshotTest {
             nextEpisode = null,
             nextAiringAt = null,
             franchise = emptyList(),
+            // What the engine would pick for these fixtures: the part-watched episode.
+            continueIndex = seasons.flatMap { it.episodes }.indexOfFirst { it.resumeAt != null }
+                .takeIf { it >= 0 }?.toUInt(),
         )
 
         val SERIES = title(