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.

crates/pstr-app/src/app.rs

@@ -1312,9 +1312,18 @@ impl App {
             })?;
             Some((title.name.clone(), episode.watch))
         });
-        let Some((name, Some(before))) = found else {
+        let Some((name, before)) = found else {
             return;
         };
+        // An episode Continue watching offers next has never been played.
+        // Its undo is a reset as old as time, which the finished episode
+        // before it outranks.
+        let before = before.unwrap_or(pstr_core::catalog::WatchState {
+            position_secs: 0.0,
+            duration_secs: None,
+            watched: false,
+            updated_at: 0,
+        });
         self.engine.save_watch_state(
             share_id.clone(),
             link_id.clone(),

crates/pstr-app/src/ui/library.rs

@@ -382,7 +382,7 @@ fn tile_shape(tiles: Tiles) -> (f32, f32) {
     }
 }
 
-/// The top shelf: one card per part-watched title, scrolling sideways.
+/// The top shelf: one card per title in progress, scrolling sideways.
 fn continue_row(
     ui: &mut egui::Ui,
     art: &mut Art<'_>,
@@ -395,21 +395,17 @@ fn continue_row(
         .show(ui, |ui| {
             ui.horizontal(|ui| {
                 for title in titles {
-                    let Some(episode) = title.resume() else {
+                    let Some(episode) = title.continue_with() else {
                         continue;
                     };
-                    let remaining = episode
-                        .watch
-                        .and_then(|watch| watch.duration_secs)
-                        .map(|duration| {
-                            format!(
-                                "{} left",
-                                ui::format_time(
-                                    duration - episode.watch.map_or(0.0, |w| w.position_secs)
-                                )
-                            )
-                        })
-                        .unwrap_or_else(|| episode.label());
+                    let remaining = match episode.resume_at() {
+                        Some(at) => episode
+                            .watch
+                            .and_then(|watch| watch.duration_secs)
+                            .map(|duration| format!("{} left", ui::format_time(duration - at)))
+                            .unwrap_or_else(|| episode.label()),
+                        None => format!("Next: {}", episode.label()),
+                    };
 
                     let picture = art.of(title);
                     let response = ui::card(
@@ -550,7 +546,7 @@ fn tile_menu(
             });
         }
         if continuing
-            && let Some(episode) = title.resume()
+            && let Some(episode) = title.continue_with()
             && ui
                 .button("Remove from Continue watching")
                 .on_hover_text("Forget where you stopped in it")