Fix: Implement touch events in violent theremin demo

[mattfsourcecode]

This PR resolves issue #126 by enabling touch events in the Violent Theremin demo, making the example usable on mobile device web browsers.

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Changes Introduced in This PR:

1. Replaces <p> with <button> for the start message:

   • Mobile web browsers enforce stricter protocols for requiring an AudioContext to be initialized via a user gesture, such as a button click. This migration allows the AudioContext to run on mobile devices and avoids the following console warning:
     
   • The .start-message button is styled to cover the full screen, closely mimicking the previous <p> element while keeping the text centered.

2. Prevents duplicate init() invocations:

   • Ensures the init() method is called only once by removing redundant keydown event listeners and triggering init() exclusively on the .start-message button click.
   • Since the init() method already configures keyboard controls, removing window.addEventListener("keydown", init); did not impact the functionality.
   • This change aligns with feedback from @normanr in Add support for touch events to violent theremin demo sprints#3937 (comment).

3. Prevents scrolling on touchmove events:

   • Disables scrolling caused by touch gestures to maintain functionality on touch screens.
   • Registers the updatePage() method as a callback for touchmove events within the init() method.

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This PR allows the example to function seamlessly on both desktop and mobile browsers, providing a consistent and smooth experience across devices.