fix(android): pin Slider inactiveTrackColor to surfaceVariant (parity)

Material3 changed the Slider's default inactiveTrackColor to colorScheme.secondaryContainer, which MinstrelTheme does not override -- so M3's baseline purple leaked into both the mini scrubber and the NowPlaying ScrubberRow. Flutter explicitly sets inactiveColor = fs.slate; mirror that by pinning inactiveTrackColor to surfaceVariant (which the theme already maps to slate).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/player/ui/MiniPlayer.kt

@@ -190,6 +190,10 @@ private fun MiniSeekBar(positionMs: Long, durationMs: Long, onSeek: (Long) -> Un
         colors = SliderDefaults.colors(
             thumbColor = MaterialTheme.colorScheme.primary,
             activeTrackColor = MaterialTheme.colorScheme.primary,
+            // M3 default inactiveTrackColor is secondaryContainer, which the
+            // theme doesn't override — that's where the M3-baseline purple
+            // leaks in. Pin to surfaceVariant (= slate) for Flutter parity.
+            inactiveTrackColor = MaterialTheme.colorScheme.surfaceVariant,
         ),
     )
 }

android/app/src/main/java/com/fabledsword/minstrel/player/ui/NowPlayingScreen.kt

@@ -409,6 +409,9 @@ private fun ScrubberRow(positionMs: Long, durationMs: Long, onSeek: (Long) -> Un
         colors = SliderDefaults.colors(
             thumbColor = accent,
             activeTrackColor = accent,
+            // M3 inactive default is secondaryContainer (purple baseline);
+            // pin to surfaceVariant (= slate) for Flutter parity.
+            inactiveTrackColor = MaterialTheme.colorScheme.surfaceVariant,
         ),
     )
     Row(