feat: Smart play/pause overlay on DetailPanel album art

DetailPanel.tsx:
- Added isPlaying and currentClipId props to track playback state
- Added onTogglePlay prop for pausing currently-playing track
- Album art click handler now smart:
  * If this track IS playing → pause it (onTogglePlay)
  * If this track is NOT playing → play it (onPlay)
- Hover overlay icon adapts dynamically:
  * Currently playing + hover → shows PAUSE icon (⏸)
  * Not playing + hover → shows PLAY icon (▶)
- Works for both artwork images and placeholder state

App.tsx:
- Passes isPlaying, currentClipId, and onTogglePlay to DetailPanel

Visual states:
- Different track in panel: ▶ on hover (click to play)
- Same track playing: ⏸ on hover (click to pause)
- Same track paused: ▶ on hover (click to resume)

frontend/src/App.tsx

@@ -229,12 +229,15 @@ function App() {
       {selectedClip && (
         <DetailPanel
           clip={selectedClip}
+          isPlaying={isPlaying}
+          currentClipId={currentClip?.id}
           onClose={() => setSelectedClip(null)}
           onRename={handleRename}
           onDelete={handleDelete}
           onUploadArtwork={handleUploadArtwork}
           onRemoveArtwork={handleRemoveArtwork}
           onPlay={handlePlay}
+          onTogglePlay={() => setIsPlaying(!isPlaying)}
         />
       )}