Add always-visible player, play button on sidebar thumbnail, and GitHub link

- Player: Show minimal placeholder when no song is playing instead of hiding
- RightSidebar: Add play button overlay on cover art thumbnail
  - Hover to reveal play/pause button
  - Click anywhere on cover to play
  - Shows pause icon when song is currently playing
- SettingsModal: Add GitHub repo link for issues and feature requests

App.tsx

@@ -959,6 +959,9 @@ export default function App() {
                   onNavigateToSong={handleNavigateToSong}
                   isLiked={selectedSong ? likedSongIds.has(selectedSong.id) : false}
                   onToggleLike={toggleLike}
+                  onPlay={playSong}
+                  isPlaying={isPlaying}
+                  currentSong={currentSong}
                 />
               </div>
             )}
@@ -1086,6 +1089,9 @@ export default function App() {
               onNavigateToSong={handleNavigateToSong}
               isLiked={selectedSong ? likedSongIds.has(selectedSong.id) : false}
               onToggleLike={toggleLike}
+              onPlay={playSong}
+              isPlaying={isPlaying}
+              currentSong={currentSong}
             />
           </div>
         </div>