Add global volume control slider

[Curucail]

This pull request introduces a master volume control feature to the TrackSwitch player #17 .

New volume control feature:

• Added a volume slider and icon to the main player UI, allowing users to adjust the master volume (js/trackswitch.js, css/trackswitch.css). [1] [2]
• Implemented a new gain node (gainNodeVolume) in the Web Audio API signal chain to control output volume, connecting it between the master gain node and the audio context destination (js/trackswitch.js). [1] [2]

Event handling and UI updates:

• Added event handlers to update the audio volume in real time as the slider moves, and to prevent volume slider interactions from interfering with other player controls (js/trackswitch.js).
• Updated the volume icon dynamically to reflect the current volume level (muted, low, or high) for better user feedback (js/trackswitch.js).

Responsive and style adjustments:

• Added and adjusted CSS for the volume control and icon, including responsive tweaks for different screen sizes (css/trackswitch.css). [1] [2]

Screenshot: