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/css-live-reload.js

@@ -0,0 +1,42 @@
+// CSS Live Reload — polls /theme.css and hot-swaps the stylesheet on change.
+// No full page reload needed. Included automatically in dev pages.
+(function() {
+  'use strict';
+  var POLL_MS = 500;
+  var link = document.querySelector('link[rel="stylesheet"][href*="theme"]');
+  if (!link) return;
+
+  var prevHash = 0;
+
+  function djb2(str) {
+    var h = 5381;
+    for (var i = 0; i < str.length; i++) {
+      h = ((h << 5) + h + str.charCodeAt(i)) | 0;
+    }
+    return h;
+  }
+
+  function check() {
+    fetch('/theme.css', { cache: 'no-store' })
+      .then(function(r) { return r.text(); })
+      .then(function(css) {
+        var h = djb2(css);
+        if (prevHash && h !== prevHash) {
+          // Clone link and swap — avoids FOUC
+          var next = link.cloneNode();
+          next.href = '/theme.css?t=' + Date.now();
+          link.parentNode.insertBefore(next, link.nextSibling);
+          next.onload = function() {
+            link.parentNode.removeChild(link);
+            link = next;
+          };
+          console.log('[css-live-reload] theme.css updated');
+        }
+        prevHash = h;
+      })
+      .catch(function() { /* ignore network errors */ });
+  }
+
+  check();
+  setInterval(check, POLL_MS);
+})();