feat(ui): round icon-only search button in the title bar

The shortcut moves from the visible ⌘K badge into the tooltip.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -832,11 +832,11 @@ export const App: React.FC = () => {
             <button
               type="button"
               onClick={() => setShowCommandPalette(true)}
-              className="h-8 px-2.5 rounded-btn bg-panel border border-border text-muted hover:text-main flex items-center gap-1.5 text-xs transition-colors cursor-pointer shrink-0"
-              title="Globale Suche öffnen (Cmd+K oder /)"
+              className="w-8 h-8 rounded-full bg-panel border border-border text-muted hover:text-main flex items-center justify-center transition-colors cursor-pointer shrink-0"
+              title="Globale Suche (Strg+K / ⌘K oder /)"
+              aria-label="Globale Suche (Strg+K / ⌘K oder /)"
             >
               <Search className="w-3.5 h-3.5" />
-              <span className="hidden lg:inline font-mono text-[10px] text-accent font-semibold">⌘K</span>
             </button>
           </div>