feat: Add chapter-relative MediaSession position state

When "Use chapter track" is enabled, the Media Session API now reports
duration and position relative to the current chapter instead of the
full audiobook. This makes the OS lock screen/notification scrubber
span only the current chapter.

- Call setPositionState() with chapter-relative values
- Map OS seek requests back to absolute file position
- Update metadata with chapter title on chapter change
- Fall back to full-file behavior when setting is disabled

client/components/app/MediaPlayerContainer.vue

@@ -148,6 +148,10 @@ export default {
     currentChapter() {
       return this.chapters.find((chapter) => chapter.start <= this.currentTime && this.currentTime < chapter.end)
     },
+    useChapterTrack() {
+      const _useChapterTrack = this.$store.getters['user/getUserSetting']('useChapterTrack') || false
+      return this.chapters.length ? _useChapterTrack : false
+    },
     title() {
       if (this.playerHandler.displayTitle) return this.playerHandler.displayTitle
       return this.mediaMetadata.title || 'No Title'
@@ -291,6 +295,8 @@ export default {
     setPlaybackRate(playbackRate) {
       this.currentPlaybackRate = playbackRate
       this.playerHandler.setPlaybackRate(playbackRate)
+      // Update position state with new playback rate
+      this.updateMediaSessionPositionState()
     },
     seek(time) {
       this.playerHandler.seek(time)
@@ -300,6 +306,7 @@ export default {
       this.playerHandler.seek(time, false)
     },
     setCurrentTime(time) {
+      const previousChapterId = this.currentChapter?.id
       this.currentTime = time
       if (this.$refs.audioPlayer) {
         this.$refs.audioPlayer.setCurrentTime(time)
@@ -308,6 +315,14 @@ export default {
       if (this.sleepTimerType === this.$constants.SleepTimerTypes.CHAPTER && this.sleepTimerSet) {
         this.checkChapterEnd()
       }
+
+      // Update MediaSession position state (chapter-relative when useChapterTrack enabled)
+      this.updateMediaSessionPositionState()
+
+      // If chapter changed and useChapterTrack enabled, update MediaSession metadata
+      if (this.useChapterTrack && this.currentChapter?.id !== previousChapterId && this.currentChapter) {
+        this.updateMediaSessionForChapter()
+      }
     },
     setDuration(duration) {
       this.totalDuration = duration
@@ -355,7 +370,20 @@ export default {
     mediaSessionSeekTo(e) {
       console.log('Media session seek to', e)
       if (e.seekTime !== null && !isNaN(e.seekTime)) {
-        this.playerHandler.seek(e.seekTime)
+        // When "Use chapter track" is enabled and chapters exist, seekTime is
+        // relative to current chapter start. Map it back to absolute position.
+        if (this.useChapterTrack && this.currentChapter) {
+          const chapterStart = this.currentChapter.start
+          const chapterEnd = this.currentChapter.end
+          const chapterDuration = chapterEnd - chapterStart
+          // Clamp seekTime to chapter bounds to prevent seeking outside chapter
+          const clampedSeekTime = Math.max(0, Math.min(e.seekTime, chapterDuration))
+          const absoluteTime = chapterStart + clampedSeekTime
+          this.playerHandler.seek(absoluteTime)
+        } else {
+          // "Use chapter track" disabled or no chapters - use full-file seek
+          this.playerHandler.seek(e.seekTime)
+        }
       }
     },
     mediaSessionPreviousTrack() {
@@ -373,6 +401,91 @@ export default {
         navigator.mediaSession.playbackState = this.isPlaying ? 'playing' : 'paused'
       }
     },
+    /**
+     * Update MediaSession metadata when chapter changes (only if useChapterTrack is enabled).
+     * Updates the title to show chapter info and resets position state.
+     */
+    updateMediaSessionForChapter() {
+      if (!('mediaSession' in navigator) || !this.useChapterTrack || !this.currentChapter) {
+        return
+      }
+
+      // Update metadata with chapter title
+      const baseTitle = this.title
+      const chapterTitle = this.currentChapter.title
+
+      navigator.mediaSession.metadata = new MediaMetadata({
+        title: chapterTitle || baseTitle,
+        artist: this.playerHandler.displayAuthor || this.mediaMetadata.authorName || 'Unknown',
+        album: baseTitle,
+        artwork: [
+          {
+            src: this.$store.getters['globals/getLibraryItemCoverSrc'](this.streamLibraryItem, '/Logo.png', true)
+          }
+        ]
+      })
+
+      // Immediately update position state for new chapter
+      this.updateMediaSessionPositionState()
+    },
+    /**
+     * Update MediaSession position state.
+     * When "Use chapter track" is enabled and a chapter is active, reports
+     * duration/position relative to chapter bounds so the OS scrubber spans
+     * only the current chapter. Otherwise uses full-file duration/position.
+     */
+    updateMediaSessionPositionState() {
+      if (!('mediaSession' in navigator) || !navigator.mediaSession.setPositionState) {
+        return
+      }
+
+      const playbackRate = this.currentPlaybackRate || 1
+
+      if (this.useChapterTrack && this.currentChapter) {
+        // "Use chapter track" enabled - report chapter-relative values
+        const chapterStart = this.currentChapter.start
+        const chapterEnd = this.currentChapter.end
+        const chapterDuration = chapterEnd - chapterStart
+
+        // Calculate position relative to chapter start
+        // Clamp to valid range to handle slight timing drift
+        let chapterPosition = this.currentTime - chapterStart
+        chapterPosition = Math.max(0, Math.min(chapterPosition, chapterDuration))
+
+        // Validate values to prevent NaN or invalid states
+        if (isNaN(chapterDuration) || chapterDuration <= 0 || isNaN(chapterPosition)) {
+          console.warn('Invalid chapter position state values, skipping update')
+          return
+        }
+
+        try {
+          navigator.mediaSession.setPositionState({
+            duration: chapterDuration,
+            position: chapterPosition,
+            playbackRate: playbackRate
+          })
+        } catch (e) {
+          console.error('Error setting media session position state:', e)
+        }
+      } else if (this.totalDuration > 0) {
+        // "Use chapter track" disabled or no chapters - use full-file values
+        const position = Math.max(0, Math.min(this.currentTime, this.totalDuration))
+
+        if (isNaN(this.totalDuration) || isNaN(position)) {
+          return
+        }
+
+        try {
+          navigator.mediaSession.setPositionState({
+            duration: this.totalDuration,
+            position: position,
+            playbackRate: playbackRate
+          })
+        } catch (e) {
+          console.error('Error setting media session position state:', e)
+        }
+      }
+    },
     setMediaSession() {
       if (!this.streamLibraryItem) {
         console.error('setMediaSession: No library item set')