Adds add to favorites button in SongRow

frontend/src/api/player.tsx

@@ -142,28 +142,18 @@ export const API = {
     return response.json();
   },
 
-  async addToFavorites(url: string): Promise<void> {
-    // Maybe a little weird to make an empty PlaylistItem here, but we do want to support adding
-    // known PlaylistItems to favorites in the future. 
-    const playlistItem: PlaylistItem = {
-      filename: url,
-      title: null,
-      id: 0,
-      playing: null,
-      metadata: undefined,
-    };
-
+  async addToFavorites(filename: string): Promise<void> {
     await fetch('/api/favorites', {
       method: 'POST',
       headers: {
         'Content-Type': 'application/json',
       },
-      body: JSON.stringify(playlistItem),
+      body: JSON.stringify({ filename }),
     });
   },
 
-  async removeFromFavorites(id: number): Promise<void> {
-    await fetch(`/api/favorites/${id}`, { method: 'DELETE' });
+  async removeFromFavorites(filename: string): Promise<void> {
+    await fetch(`/api/favorites/${encodeURIComponent(filename)}`, { method: 'DELETE' });
   },
 
   async clearFavorites(): Promise<void> {