ui: adjust toolbar heights and icon sizes, remove duplicate AI icon

- TopBar: h-16 → h-[58px] (10% lower), icons w-5 → w-4
- PluginToolbar: min-h-[36px] → min-h-[43px] (20% higher), icons w-3.5 → w-4
- Remove duplicate AI computer icon from TopBar (AISidebar collapsed icon remains)
- Clean up unused imports (toggleAISidebar, aiSidebarCollapsed)

frontend/src/components/layout/AISidebar.tsx

@@ -41,7 +41,7 @@ export function AISidebar() {
           aria-label="KI Assistent öffnen"
           title="KI Assistent"
         >
-          <svg className="w-5 h-5 text-secondary-600" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">
+          <svg className="w-4 h-4 text-secondary-600" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">
             <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M9.75 17L9 20l-1 1h8l-1-1-.75-3M3 13h18M5 17h14a2 2 0 002-2V5a2 2 0 00-2-2H5a2 2 0 00-2 2v10a2 2 0 002 2z" />
           </svg>
         </button>