Fixed bug when running yarn dev on Windows 11

[shaunroselt]

This fixes ERROR Unable to activate the theme "casper". and Unable to activate the theme "casper". when running yarn dev on Windows 11 mentioned on this issue: #17704

This commit doesn't fix the entire issue, but it fixes the first part of the issue. It's also only tested on my Windows 11 system. Someone just needs to test it on MacOS and make sure it's working there too, but I'm 99% sure it won't cause any issues on MacOS and will just work as always.

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🤖 Generated by Copilot at 47a7921

Fix admin panel loading bug by ensuring destination directory for index.html exists. Refactor AdminAuthAssetsService.js to use variables for paths.

[shaunroselt]

Just for some added info here. For some reason on Windows 11, it kept failing on fs.copyFile because the FileDestination directories didn't exist. On MacOS, it probably created the directories if it doesn't exist with the fs.copyFile function, but on my Windows 11 Build, this didn't happen. So it kept failing.

Thus, I just needed to add fs.mkdir before fs.copyFile and actually create the directories before trying to copy to them.

So now, this error won't happen anymore on Windows 11: