treewide: resolve testbed runtime inconveniences

[trueNAHO]

The following testbed runtime (nix run .#testbed:<TESTBED>:<THEME>) inconveniences should be resolved:

• Emacs

  • [ ] Color scheme unapplied by default on startup.

• GRUB

  • [ ] Test bootloader

• Limine

  • [ ] Test bootloader

• VSCode

  • [ ] Color scheme unapplied by default on startup.
  • [ ] Application takes way too long to start.

• Zed

  • [ ] Color scheme unapplied by default on startup.
  • [ ] Startup notification should be disabled.

This list is very likely incomplete. Feel free to mention other things to be added to the list.