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/hiccup/src/dev/hiccup.clj

@@ -503,6 +503,7 @@
           [:script (h/raw theme-persistence-script)]]
          [:body
           [:script {:src "/theme-adapter.js" :defer true}]
+          [:script {:src "/css-live-reload.js" :defer true}]
           (sidebar/sidebar-layout {}
             (app-sidebar active-page port)
             (sidebar/sidebar-overlay {})
@@ -534,6 +535,11 @@
        :headers {"Content-Type" "application/javascript"}
        :body (slurp "dev/theme-adapter.js")}
 
+      (= path "/css-live-reload.js")
+      {:status 200
+       :headers {"Content-Type" "application/javascript"}
+       :body (slurp "dev/css-live-reload.js")}
+
       (resolve-page path)
       {:status 200
        :headers {"Content-Type" "text/html; charset=utf-8"}