Fix: chat panel now starts minimized and collapses

The panel's display:flex overrode the browser's default [hidden] styling,
so it rendered open from first paint and the close button was inert. Added
a global [hidden]{display:none!important} guard so the attribute always
wins. Browser-verified: loads minimized, opens, closes, toggles. Assets
v=20260904l.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

public/assets/site.css

@@ -14,6 +14,7 @@
   --radius:18px;
 }
 *{box-sizing:border-box}
+[hidden]{display:none!important} /* beats any display: set by a class (chat panel bug) */
 html{scroll-behavior:smooth}
 body{margin:0;background:var(--ground);color:var(--ink);font:16px/1.65 "Segoe UI",system-ui,sans-serif;overflow-x:hidden}
 /* topo-contour ground texture, very faint */