Enhance Physics API and Fix Loading Screen Stacking

[brettfxio]

This PR introduces several enhancements to the physics API and fixes a UX bug related to the loading screen.

Physics Enhancements

• Velocity Control: Added setLinearVelocity and setAngularVelocity to the BabylonSceneManager, allowing developers to directly set the velocity of physics-enabled objects. Corresponding Blockly blocks have been added to the 'Game' category.
• Collision Management: Added a clearCollisionCache method and block. This is useful for scenarios where a collision needs to be detected multiple times between the same pair of objects (e.g., a repeating bounce effect).

UX Bug Fixes

• Loading Screen Stacking: Fixed an issue where multiple calls to displayLoadingUI (either from code or rapid user interaction) would result in multiple div elements being appended to the DOM. A guard check now ensures only one loading screen exists at a time.

Testing

• Added tests/loading_screen.spec.js to specifically verify the loading screen fix.
• Verified all other critical and feature tests pass.
• Visually verified the new blocks and the loading screen behavior using Playwright screenshots.

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PR created automatically by Jules for task 13312066207241550192 started by @brettfxio

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