Add dark mode support and replace group assignment with playlist assignment

- Added comprehensive dark mode styling to all pages:
  * Dashboard (workflow guide, secondary text, log items)
  * Admin panel with user management system
  * Content/playlist management page
  * Upload media page
  * Add player page

- Implemented user management system:
  * Create/edit/delete users
  * Two role types (user/admin)
  * Reset password functionality
  * Role-based permissions

- Replaced group assignment with playlist assignment:
  * Players now directly assigned to playlists
  * Updated add player form and backend
  * Removed group selection from player creation

- Fixed bugs:
  * Updated instance_path configuration for SQLite
  * Fixed import path in app factory
  * Updated dependencies (Pillow 11.0.0, removed gevent)

- Added start.sh script for easy development server launch

app/app.py

@@ -23,7 +23,9 @@ def create_app(config_name=None):
     Returns:
         Flask application instance
     """
-    app = Flask(__name__)
+    # Set instance path to absolute path
+    instance_path = os.path.abspath(os.path.join(os.path.dirname(__file__), '..', 'instance'))
+    app = Flask(__name__, instance_path=instance_path, instance_relative_config=True)
     
     # Load configuration
     if config_name == 'production':
@@ -129,7 +131,7 @@ def register_commands(app):
     @click.option('--password', prompt=True, hide_input=True, help='Admin password')
     def create_admin(username, password):
         """Create an admin user"""
-        from models.user import User
+        from app.models.user import User
         
         # Check if user exists
         if User.query.filter_by(username=username).first():