refactor: adopt shared Button component in 18 modal components

Replace raw <button> elements with <Button> from ui/Button.tsx:
- Primary buttons (bg-gradient-brand) → <Button variant="primary">
- Secondary buttons (border-border) → <Button variant="secondary">
- Ghost buttons → <Button variant="ghost">
- Loading states use loading prop instead of manual Loader2 spinner

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/step-library/StepDetailModal.tsx

@@ -4,6 +4,7 @@ import { cn } from '@/lib/utils'
 import { MarkdownContent } from '@/components/ui/MarkdownContent'
 import { stepsApi } from '@/api/steps'
 import type { Step, Review } from '@/types/step'
+import { Button } from '@/components/ui/Button'
 
 interface StepDetailModalProps {
   stepId: string
@@ -318,22 +319,20 @@ export function StepDetailModal({ stepId, onClose, onInsert }: StepDetailModalPr
 
         {/* Footer - Actions */}
         <div className="flex gap-2 border-t border-border p-4">
-          <button
+          <Button
+            variant="secondary"
             onClick={onClose}
-            className="flex-1 rounded-md border border-border px-4 py-2 text-sm font-medium text-muted-foreground hover:bg-accent hover:text-foreground"
+            className="flex-1"
           >
             Cancel
-          </button>
-          <button
+          </Button>
+          <Button
             onClick={handleInsert}
             disabled={!step}
-            className={cn(
-              'flex-1 rounded-md bg-gradient-brand px-4 py-2 text-sm font-medium text-white shadow-lg shadow-primary/20',
-              'hover:opacity-90 disabled:opacity-50'
-            )}
+            className="flex-1"
           >
             Insert Into Session
-          </button>
+          </Button>
         </div>
       </div>
     </div>