refactor(styles): split theme.css into per-theme files (#656)

theme.css (16138 LOC) → 122 per-section files in src/styles/themes/ +
an index.css that imports them in original cascade order.

Concatenating all files via index.css reproduces the original byte stream
(+1 trailing newline, cosmetic).

Each top-level section header in theme.css (matching /^\/\* ─{3,}/)
becomes its own file, slugged from the header text (or the [data-theme]
selector found in the body when the header was a banner). Pure-separator
headers fold into the previous section so they don't create empty files.

Generated via /tmp/split-theme-css.mjs.

src/main.tsx

@@ -3,7 +3,7 @@ import ReactDOM from 'react-dom/client';
 import App from './App';
 import { runPreReactBootstrap } from './app/bootstrap';
 import './i18n';
-import './styles/theme.css';
+import './styles/themes/index.css';
 import './styles/layout.css';
 import './styles/components.css';
 import './styles/tracks.css';