fix: clean up chat UI layout and align theme to neutral white

- Replace beige background with clean neutral white (matches Grok aesthetic)
- Remove hardcoded hex colors in thread.tsx - use CSS variables throughout
- Remove scroll-to-bottom button that showed incorrectly after auto-send
- Chat container now integrates visually with the page instead of floating

Made-with: Cursor

components/AtlasChat.tsx

@@ -91,14 +91,14 @@ function AtlasChatInner({
   };
 
   return (
-    <div className="flex flex-col rounded-2xl overflow-hidden bg-[#fdfdfd] dark:bg-[#141414] ring-1 ring-[#e5e5e5] dark:ring-[#2a2a2a]" style={{ height: "600px" }}>
+    <div className="flex flex-col rounded-2xl overflow-hidden bg-card ring-1 ring-border" style={{ height: "600px" }}>
       {/* Minimal header bar */}
-      <div className="flex items-center justify-end px-4 py-2.5 shrink-0 border-b border-[#e5e5e5] dark:border-[#2a2a2a]">
+      <div className="flex items-center justify-end px-4 py-2.5 shrink-0 border-b border-border">
         <Button
           variant="ghost"
           size="sm"
           onClick={handleReset}
-          className="h-7 w-7 p-0 text-[#9a9a9a] dark:text-[#6b6b6b] hover:text-[#0d0d0d] dark:hover:text-white"
+          className="h-7 w-7 p-0 text-muted-foreground hover:text-foreground"
           title="Start over"
         >
           <RotateCcw className="w-3.5 h-3.5" />