ui: generous whitespace — floating compose, spacious messages + header

- Compose: floating bordered container with inner padding, no top border
- Compose: bottom bar with prefix + "enter to send" hint
- Messages: increased padding (py-3/4, px-4/5)
- Header: more breathing room (py-3/4, px-4/6)
- Modern 2026 spacing throughout

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/colony/src/App.tsx

@@ -121,7 +121,7 @@ export default function App() {
       </div>
 
       <div className="flex-1 flex flex-col min-w-0">
-        <div className="px-3 py-2 md:px-4 border-b-2 border-border flex items-center gap-2">
+        <div className="px-4 py-3 md:px-6 md:py-4 border-b-2 border-border flex items-center gap-3">
           <Sheet open={sheetOpen} onOpenChange={setSheetOpen}>
             <SheetTrigger className="md:hidden p-1 h-8 w-8 text-muted-foreground hover:text-primary font-mono font-bold text-lg">
               =