fix(ui,player): scroll restore, immersive fullscreen, watched toggle, handoff timeline, PiP

Batch of reported bugs and enhancements.

UI
- Pages no longer inherit the previous page's scroll position (DR-156, UR-072).
  The shell keeps its scrollers alive across navigation by design, so the
  element never remounts and its scrollTop survived the route change; SvelteKit
  restores window scroll, which this app never uses. ScrollMemory records the
  offset per route and per container: forward moves reset to the top, Back
  restores where the route was left.
- Season header stacks on narrow screens, and the title span gets min-w-0 so it
  actually truncates instead of overflowing under the action buttons.
- Favourites gets a labelled tile at the head of the library grid rather than
  only an unlabelled heart icon in the header.

Playback
- Full-screen video on Android hides the system bars (DR-157, UR-066).
  requestFullscreen() cannot touch the Activity window from inside a WebView, so
  the control did nothing visible while the bars stayed painted over the video.
  ImmersiveModeBridge hides them, restored on exit, Escape and teardown.
- Background-audio handoff stops leaking its relative timeline (DR-159).
  background_audio_base was a display-only correction applied in two places
  while progress reports to Jellyfin, the frontend and media3's own seeks all
  worked in the relative timeline treating it as absolute — each crossing losing
  exactly `base` seconds. The conversion now happens once, in the position tick,
  and inbound seeks resolve through seek_absolute, which re-opens the stream at
  the requested position because the handoff transcode cannot seek.
- Picture-in-picture works on the path that actually plays video (DR-160).
  canEnterPip demanded a native ExoPlayer surface, but that path is behind a
  flag defaulting to off, so PiP could never engage. It now accepts the WebView
  <video> too, keeping the WebView visible and routing play/pause to the element.
- Native video is now the default so PiP has a real surface (DR-161). The
  scrub-regression tests pinned the flag-off path implicitly; they now mock it
  off explicitly. The native scrub/seek path is not covered by the suite and
  needs device verification.

Watched state
- Watched toggle on the episode row, season header, series and movie hero, and
  the Episode Focus View (DR-158, UR-073). Both backend halves already existed
  with no caller. storage_set_watched covers a container's episodes so the
  toggle is honest offline, and QueuedOp::MarkUnplayed gives the sync queue the
  missing direction.

Release
- Fix the Android versionCode floor (set-version.sh). v0.5.2 shipped code 5002
  under an earlier minor*1000 scheme, but the current minor*100 formula yields
  1502 for that version and 1503 for 0.5.3 — so every 0.5.x release built from
  it was an un-installable downgrade for anyone already on v0.5.2. Widened to
  10000 + major*1000000 + minor*1000 + patch (0.5.3 -> 15003).
- Bump to 0.5.3.

src-tauri/src/commands/sync_drain.rs

@@ -52,6 +52,12 @@ pub enum QueuedOp {
     MarkPlayed {
         item_id: String,
     },
+    /// The inverse, queued by the watched toggle. Pushes as `clear_watch_history`
+    /// (Jellyfin's mark-unplayed), which also zeroes the resume position — so an
+    /// item un-marked offline does not come back carrying a stale position.
+    MarkUnplayed {
+        item_id: String,
+    },
     /// Legacy rows only — live favourite toggles drain via `user_data.pending_sync`
     /// (DR-120). Supported so a row written by an older build still lands.
     Favorite {
@@ -105,6 +111,7 @@ pub fn parse_queued_op(
             position_ticks: ticks(),
         }),
         "mark_played" => Ok(QueuedOp::MarkPlayed { item_id }),
+        "mark_unplayed" => Ok(QueuedOp::MarkUnplayed { item_id }),
         "mark_favorite" => Ok(QueuedOp::Favorite {
             item_id,
             is_favorite: true,
@@ -137,6 +144,7 @@ impl<T: MediaRepository + ?Sized> SyncSink for T {
                 position_ticks,
             } => self.report_playback_stopped(item_id, *position_ticks).await,
             QueuedOp::MarkPlayed { item_id } => self.mark_played(item_id).await,
+            QueuedOp::MarkUnplayed { item_id } => self.clear_watch_history(item_id).await,
             QueuedOp::Favorite {
                 item_id,
                 is_favorite,
@@ -1031,4 +1039,55 @@ mod tests {
         assert!(parse_queued_op("mark_played", None, None).is_err());
         assert!(parse_queued_op("teleport", Some("ep1"), None).is_err());
     }
+
+    /// Un-marking watched queues like marking watched does, so the toggle works
+    /// in both directions while the server is unreachable rather than only one.
+    ///
+    /// TRACES: UR-073 | DR-158 | UT-154
+    #[test]
+    fn test_parse_accepts_mark_unplayed() {
+        assert_eq!(
+            parse_queued_op("mark_unplayed", Some("ep1"), None).unwrap(),
+            QueuedOp::MarkUnplayed {
+                item_id: "ep1".to_string()
+            },
+        );
+
+        assert!(parse_queued_op("mark_unplayed", None, None).is_err());
+    }
+
+    /// The queued un-mark reaches the server as `clear_watch_history` — Jellyfin's
+    /// mark-unplayed, which also zeroes the resume position, so a series returns
+    /// to "never watched" rather than keeping a stale position.
+    ///
+    /// TRACES: UR-073 | DR-158 | UT-154
+    #[tokio::test]
+    async fn test_drain_pushes_mark_unplayed() {
+        let db = test_db();
+        seed(
+            &db,
+            &[(
+                "u1",
+                "mark_unplayed",
+                "ep9",
+                None,
+                "pending",
+                0,
+                "2026-08-01T10:00:00Z",
+            )],
+        )
+        .await;
+
+        let sink = RecordingSink::new();
+        let report = drain_sync_queue(&db, &sink, "u1").await.unwrap();
+
+        assert_eq!(
+            sink.calls(),
+            vec![QueuedOp::MarkUnplayed {
+                item_id: "ep9".to_string()
+            }],
+        );
+        assert_eq!(report.pushed, 1);
+        assert_eq!(report.remaining, 0);
+    }
 }