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/PluginToolbar.tsx

@@ -14,7 +14,7 @@ function ToolbarButton({ item }: { item: ToolbarItem }) {
       `}
       title={item.label}
     >
-      {item.icon && <span className="w-3.5 h-3.5 flex-shrink-0">{item.icon}</span>}
+      {item.icon && <span className="w-4 h-4 flex-shrink-0">{item.icon}</span>}
       <span className="hidden sm:inline">{item.label}</span>
     </button>
   );
@@ -66,7 +66,7 @@ export function PluginToolbar() {
 
   return (
     <div
-      className="flex items-center gap-1 px-2 sm:px-3 py-1 bg-white border-b border-secondary-200 min-h-[36px] overflow-x-auto"
+      className="flex items-center gap-1 px-2 sm:px-3 py-1.5 bg-white border-b border-secondary-200 min-h-[43px] overflow-x-auto"
       data-testid="plugin-toolbar"
     >
       {groupNames.map((groupName, gi) => (