fix(android player): seek broke after seamless track transitions (durationMs stuck at 0)

After skipping through already-buffered tracks the player stays READY, so
onPlaybackStateChanged never fires and syncState() isn't re-called; ctrl.duration
was TIME_UNSET(→0) at the transition, leaving state.durationMs stuck at 0. The
seek bar then computed fraction*0 = 0 and snapped playback to the start every time
(or to a stale value → "only 10s"). Restarting the app re-read the real duration,
which is why it recovered.

Fix: durationMs now self-heals — the 500ms ticker refreshes it too, and
resolveDurationMs() falls back to the current song's known duration while
ctrl.duration is still unset. Also record the Erkunden swipe-between-tabs idea.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

android/app/src/main/kotlin/com/novasonic/app/player/PlayerController.kt

@@ -227,11 +227,25 @@ class PlayerController(private val context: Context, private val repo: MusicRepo
             while (isActive) {
                 delay(500)
                 val ctrl = controller ?: break
-                _state.update { it.copy(positionMs = ctrl.currentPosition) }
+                // Refresh durationMs too — after a SEAMLESS track transition (skipping through
+                // already-buffered tracks) the player stays READY, so onPlaybackStateChanged never
+                // fires and syncState() isn't re-called; ctrl.duration was TIME_UNSET(→0) at the
+                // transition, leaving durationMs stuck at 0. Seek then computes fraction*0 = 0 and
+                // snaps to the start every time. Re-reading here lets it self-heal within 500ms.
+                _state.update { it.copy(positionMs = ctrl.currentPosition, durationMs = resolveDurationMs(ctrl)) }
             }
         }
     }
 
+    /** Player duration, falling back to the current song's known duration while ctrl.duration is
+     *  still TIME_UNSET (right after a media transition). Prevents a 0-duration seek → snap-to-0. */
+    private fun resolveDurationMs(ctrl: MediaController): Long {
+        val d = ctrl.duration
+        if (d > 0) return d
+        val songDurSecs = queue.getOrNull(ctrl.currentMediaItemIndex)?.duration?.toLong() ?: 0L
+        return songDurSecs * 1000L
+    }
+
     private fun stopTicker() {
         tickerJob?.cancel()
         tickerJob = null
@@ -271,7 +285,7 @@ class PlayerController(private val context: Context, private val repo: MusicRepo
             currentSong  = queue.getOrNull(idx),
             currentIndex = idx,
             positionMs   = ctrl.currentPosition,
-            durationMs   = ctrl.duration.coerceAtLeast(0),
+            durationMs   = resolveDurationMs(ctrl),
             queue        = queue,
             repeatMode   = ctrl.repeatMode,
             shuffled     = isManuallyShuffled,