fix(mobile): collapse search bar to icon on mobile

On mobile (<640px), the full search bar with placeholder text and ⌘K
badge was taking too much space in the topbar. Now shows just a
magnifying glass icon that opens the same command palette on tap.

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

frontend/src/components/layout/TopBar.tsx

@@ -78,10 +78,10 @@ export function TopBar() {
         {/* Spacer - push search to center */}
         <div className="flex-1" />
 
-        {/* Search trigger */}
+        {/* Search trigger — icon on mobile, full bar on desktop */}
         <button
           onClick={() => setCommandPaletteOpen(true)}
-          className="relative w-full text-left"
+          className="hidden sm:relative sm:block w-full text-left"
           style={{ maxWidth: '480px' }}
         >
           <Search size={16} className="absolute left-3 top-1/2 -translate-y-1/2 text-muted-foreground" />
@@ -92,6 +92,13 @@ export function TopBar() {
             {navigator.platform?.toLowerCase().includes('mac') ? '⌘K' : 'Ctrl+K'}
           </span>
         </button>
+        <button
+          onClick={() => setCommandPaletteOpen(true)}
+          className="sm:hidden rounded-lg p-2 text-muted-foreground hover:bg-card hover:text-foreground transition-colors"
+          title="Search"
+        >
+          <Search size={18} />
+        </button>
 
         {/* Spacer - push actions to right */}
         <div className="flex-1" />