Enhance Asset URL Security and Eliminate Potential DOM XSS

[brettfxio]

Revises isValidAssetURL to explicitly block protocol-relative URLs (//) which previously bypassed HTTPS requirements. Additionally, updates CustomLoadingScreen to use textContent rather than innerHTML to prevent DOM XSS and HTML injection risks. Adds a protocol-relative URL validation test to the test suite.

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PR created automatically by Jules for task 2248667335857237984 started by @brettfxio

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