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 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 avoids the following console warning:
  
• The .start-message button is styled to cover the full screen, closely mimicking the previous <p> element while centering the text.

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.
• NOTE: Removing window.addEventListener("keydown", init); did not impact the functionality since init() already configures keyboard controls.
• 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.

4. Applies global styling changes to avoid unintended interactions on mobile devices:

• Prevents element selection using the user-select property.
• Prevents pull-to-refresh effect using the overscroll-behavior property.

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

[mattfsourcecode]

Another point worth considering is that tests in various emulated mobile browsers using iOS Simulator and Android Studio produced sound on many versions of mobile devices. However, on the iPhone Xs (2018) running iOS 17.2 in Safari, there was no sound output. This suggests that AudioContext may be unreliable on older mobile devices, and none of the workarounds I tried were able to resolve the issue. This could be due to several factors, such as the device, OS, browsers and versions, or potentially an issue with the Simulator app.

The solution committed works well with newer systems, but it might be worth considering a few options: noting these potential discrepancies in the README, testing other demos to find a possible workaround for 2018 systems, or leaving it as is. Either way, it could be useful to create a new issue in this repo to track this problem. I'd be willing to help by either investigating a solution for AudioContext on older mobile devices or documenting it as a known issue in the README.