feat(android): scrubber thumb pill — fixes off-center perception

User report: the round dot didn't read as vertically centered on
the 4dp track even though geometrically it was (M3's SliderLayout
centers the track slot within the thumb's height). A small circle
on a thin horizontal bar is a known perceptual offset — the eye
expects the bar to bisect the circle, but the circle's mass extends
above and below in equal amounts the brain reads as a lift.

Swap the 14dp circle for a 4dp x 18dp vertical pill (CircleShape
on a non-square Box renders as a stadium). Same width as the track,
clearly taller — the bar visibly passes through the pill's
horizontal axis with no ambiguity. Also aligns with M3 expressive's
new vertical-handle slider direction.

Updates the ScrubTrack docstring that still referenced the prior
14dp-on-4dp pairing.

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

@@ -88,6 +88,8 @@ private const val PLAY_PAUSE_ICON_DP = 56
 private const val POP_GRACE_MS = 500L
 private val SCRUB_TRACK_HEIGHT_DP = 4.dp
 private val SCRUB_TRACK_CORNER_DP = 2.dp
+private val SCRUB_THUMB_WIDTH_DP = 4.dp
+private val SCRUB_THUMB_HEIGHT_DP = 18.dp
 
 // Vertical drag-down threshold (in pixels) past which the gesture
 // pops the player. Matches Flutter's 80px threshold in spirit;
@@ -478,15 +480,22 @@ private fun ScrubberRow(positionMs: Long, durationMs: Long, onSeek: (Long) -> Un
         modifier = Modifier.fillMaxWidth(),
         colors = sliderColors,
         interactionSource = interactionSource,
-        // Plain filled circle to match the web client's `<input
-        // type="range" accent-color>` thumb — flat, no state-layer
-        // ring, no border. M3's SliderDefaults.Thumb paints a state
-        // layer halo on press; we drop it for visual parity. Slider's
-        // 48dp hit slop still applies, so tapability is unchanged.
+        // Thin vertical pill (4dp wide × 18dp tall, fully rounded).
+        // A small circle on a thin horizontal bar reads as visually
+        // off-center even when geometrically aligned — the eye
+        // expects the bar to bisect the thumb but a 14dp circle's
+        // mass extends above and below in equal amounts that the
+        // brain perceives as offset. A vertical pill the same width
+        // as the track removes the ambiguity: the bar passes through
+        // the pill's horizontal axis cleanly. Also matches M3's
+        // expressive-slider handle direction. State-layer halo is
+        // still dropped for visual parity with the web scrubber.
+        // Slider's 48dp hit slop still applies, so tapability is
+        // unchanged.
         thumb = {
             Box(
                 modifier = Modifier
-                    .size(14.dp)
+                    .size(width = SCRUB_THUMB_WIDTH_DP, height = SCRUB_THUMB_HEIGHT_DP)
                     .clip(CircleShape)
                     .background(accent),
             )
@@ -513,12 +522,11 @@ private fun ScrubberRow(positionMs: Long, durationMs: Long, onSeek: (Long) -> Un
 
 /**
  * Custom 4dp rounded scrubber track. M3's default Track is 16dp tall
- * and reads as a heavy pill rather than a measurement line; making
- * the thumb visibly taller than the track (14dp thumb on a 4dp bar)
- * restores the "handle on a string" cue your eye reads as "tool,
- * draggable." Also drops M3's stop indicator dot, which the web
- * scrubber doesn't have. Extracted so [ScrubberRow] stays under
- * detekt's LongMethod ceiling.
+ * and reads as a heavy pill rather than a measurement line; the
+ * pill-thumb-on-thin-track pairing restores the "handle on a string"
+ * cue your eye reads as "tool, draggable." Also drops M3's stop
+ * indicator dot, which the web scrubber doesn't have. Extracted so
+ * [ScrubberRow] stays under detekt's LongMethod ceiling.
  */
 @Composable
 private fun ScrubTrack(fraction: Float, accent: Color) {