feat: add CSS live reload for dev setup

Two-part solution for automatic CSS updates during development:

1. `bb watch-theme` — polls src/ui/*.css and tokens.edn every 500ms,
   rebuilds dist/theme.css and copies to dev targets on change.

2. `dev/css-live-reload.js` — browser-side script that polls /theme.css
   and hot-swaps the stylesheet without a full page reload (no FOUC).

The watcher runs automatically in the hiccup tmux pane when using
`bb dev-all`. It can also be run standalone with `bb watch-theme`.

The live-reload script is included in all three dev targets (hiccup,
replicant, squint) and copied by `bb build-theme`.

dev/replicant/public/index.html

@@ -47,5 +47,6 @@
   <div id="app"></div>
   <script src="/js/main.js"></script>
   <script src="/theme-adapter.js" defer></script>
+  <script src="/css-live-reload.js" defer></script>
 </body>
 </html>