add touch swipe to browse songs on mobile

Swipe left/right on the carousel viewport to navigate songs.
50px threshold prevents accidental triggers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/features/song-select/SongCarousel.tsx

@@ -1,4 +1,4 @@
-import type { CSSProperties } from 'react'
+import { useRef, type CSSProperties } from 'react'
 import { SongCard } from './SongCard'
 import type { Song } from './types'
 
@@ -20,6 +20,28 @@ function getSignedOffset(currentIndex: number, index: number, length: number): n
 }
 
 export function SongCarousel({ songs, currentIndex, onSelectIndex }: SongCarouselProps) {
+  const touchStartX = useRef(0)
+  const swiped = useRef(false)
+
+  const handleTouchStart = (e: React.TouchEvent) => {
+    touchStartX.current = e.touches[0].clientX
+    swiped.current = false
+  }
+
+  const handleTouchMove = (e: React.TouchEvent) => {
+    if (swiped.current) return
+    const dx = e.touches[0].clientX - touchStartX.current
+    const threshold = 50
+    if (Math.abs(dx) > threshold) {
+      swiped.current = true
+      if (dx < 0) {
+        onSelectIndex(wrapIndex(currentIndex + 1, songs.length))
+      } else {
+        onSelectIndex(wrapIndex(currentIndex - 1, songs.length))
+      }
+    }
+  }
+
   const center = songs[currentIndex]
   const leftIndex = wrapIndex(currentIndex - 1, songs.length)
   const rightIndex = wrapIndex(currentIndex + 1, songs.length)
@@ -29,7 +51,12 @@ export function SongCarousel({ songs, currentIndex, onSelectIndex }: SongCarouse
 
   return (
     <section className="song-carousel" aria-label="Song carousel">
-      <div className="song-carousel__viewport" style={{ '--active-color': center.color } as CSSProperties}>
+      <div
+        className="song-carousel__viewport"
+        style={{ '--active-color': center.color } as CSSProperties}
+        onTouchStart={handleTouchStart}
+        onTouchMove={handleTouchMove}
+      >
         {displayIndexes.map((index) => {
           const offset = getSignedOffset(currentIndex, index, songs.length)
           return (