Improve UI consistency and fix various bugs

[bomberfish]

• Add standard variables for spacing and corner radii, affected by density setting
• Add previews to theme selector and browser density options
• Fix omnibar placeholder naming Google despite the engine being set otherwise
• Fix a couple bugs related to alternate layouts

[coderofsalvation]

Im trying to test this branch (as well as the original master):

$ git submodule update --init
$ pnpm i
$ pnpm build:dreamland
$ pnpm build 
> browser.js@1.0.1 build /tmp/browser.js
> rspack build

Error: ENOENT: no such file or directory, open '/tmp/browser.js/packages/scramjet/packages/core/dist/scramjet.wasm'
    at async open (node:internal/fs/promises:640:25)
    at async readFile (node:internal/fs/promises:1277:14)
    at async file:///tmp/browser.js/packages/scramjet/rspack.config.ts:230:17 {
  errno: -2,
  code: 'ENOENT',
  syscall: 'open',
  path: '/tmp/browser.js/packages/scramjet/packages/core/dist/scramjet.wasm'
}

$ node --version
v24.4.1
$ pnpm --version
10.12.1

Am I using old versions?