Fix theme initialization

[daniel-wer]

by loading theme before main.js. Regression was introduced in #7367.

The default state initialization assumes that the active theme is already set in the HTML (see this code). Initializing the theme in the HTML too late, leads to rendering issues, because the state is never set correctly (only once the theme is switched again). I'm not sure why the initialization is that hacky, maybe to avoid flickering if the theme is set too late, otherwise 🤔

URL of deployed dev instance (used for testing):

• https://fixtheme.webknossos.xyz

Steps to test:

• View a dataset in dark mode. Reload the page. The 3d viewport and the other viewport headers should be dark, not white.

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(Please delete unneeded items, merge only when none are left open)

• Updated changelog
• Considered common edge cases