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`.

scripts/watch-theme.bb

@@ -0,0 +1,44 @@
+#!/usr/bin/env bb
+;; Watch src/ui/*.css and src/theme/tokens.edn for changes.
+;; On change, rebuild dist/theme.css and copy to dev targets.
+
+(require '[babashka.fs :as fs]
+         '[clojure.java.io :as io]
+         '[clojure.string :as str]
+         '[ui.css.gen :as gen])
+
+(defn source-files []
+  (concat (map str (fs/glob "src/ui" "*.css"))
+          ["src/theme/tokens.edn"]))
+
+(defn get-mtimes []
+  (into {}
+    (keep (fn [p]
+            (when (fs/exists? p)
+              [p (fs/last-modified-time p)])))
+    (source-files)))
+
+(defn rebuild! []
+  (gen/build-theme! {:input "src/theme/tokens.edn" :output "dist/theme.css"})
+  (let [css (slurp "dist/theme.css")]
+    (spit "dev/replicant/public/theme.css" css)
+    (io/make-parents "dev/squint/public/theme.css")
+    (spit "dev/squint/public/theme.css" css)))
+
+(defn changed-filenames [prev curr]
+  (->> (keys curr)
+       (filter (fn [p] (not= (get prev p) (get curr p))))
+       (map (fn [p] (last (str/split p #"/"))))))
+
+(println "[watch-theme] Watching src/ui/*.css and src/theme/tokens.edn ...")
+(loop [prev (get-mtimes)]
+  (Thread/sleep 500)
+  (let [curr (get-mtimes)]
+    (when (not= prev curr)
+      (println (str "[watch-theme] Changed: " (str/join ", " (changed-filenames prev curr))))
+      (try
+        (rebuild!)
+        (println "[watch-theme] Rebuilt and copied.")
+        (catch Exception e
+          (println (str "[watch-theme] ERROR: " (.getMessage e))))))
+    (recur curr)))