app.py

```python
from flask import Flask, render_template, request, jsonify

app = Flask(__name__)

# Mock data for demonstration
mock_playlist = [
    {
        "id": 1,
        "title": "Midnight City",
        "artist": "M83",
        "duration": "4:03",
        "thumbnail": "http://static.photos/music/200x200/1",
        "downloaded": True
    },
    {
        "id": 2,
        "title": "Blinding Lights",
        "artist": "The Weeknd",
        "duration": "3:20",
        "thumbnail": "http://static.photos/music/200x200/2",
        "downloaded": True
    }
]

mock_search_results = [
    {
        "id": 3,
        "title": "Save Your Tears",
        "artist": "The Weeknd",
        "duration": "3:35",
        "thumbnail": "http://static.photos/music/200x200/3",
        "downloaded": False
    },
    {
        "id": 4,
        "title": "Starboy",
        "artist": "The Weeknd ft. Daft Punk",
        "duration": "3:50",
        "thumbnail": "http://static.photos/music/200x200/4",
        "downloaded": False
    }
]

@app.route('/')
def index():
    return render_template('player.html')

@app.route('/api/playlist')
def get_playlist():
    return jsonify(mock_playlist)

@app.route('/api/search', methods=['POST'])
def search():
    query = request.json.get('query', '')
    # In a real app, you would search your database or API here
    return jsonify(mock_search_results)

@app.route('/api/add_to_playlist', methods=['POST'])
def add_to_playlist():
    song_id = request.json.get('id')
    # Find the song in search results and add to playlist
    for song in mock_search_results:
        if song['id'] == song_id:
            new_song = song.copy()
            new_song['downloaded'] = False
            mock_playlist.append(new_song)
            return jsonify({"success": True})
    return jsonify({"success": False}), 404

@app.route('/api/remove_from_playlist', methods=['POST'])
def remove_from_playlist():
    song_id = request.json.get('id')
    global mock_playlist
    mock_playlist = [song for song in mock_playlist if song['id'] != song_id]
    return jsonify({"success": True})

if __name__ == '__main__':
    app.run(debug=True)
```