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/library/ForkModal.tsx

@@ -4,6 +4,7 @@ import { treesApi } from '@/api/trees'
 import { toast } from '@/lib/toast'
 import { cn } from '@/lib/utils'
 import { useNavigate } from 'react-router-dom'
+import { Button } from '@/components/ui/Button'
 
 interface ForkModalProps {
   treeId: string
@@ -116,20 +117,20 @@ export function ForkModal({ treeId, treeName, onClose }: ForkModalProps) {
 
           {/* Footer */}
           <div className="flex justify-end gap-2 pt-1">
-            <button
+            <Button
               type="button"
+              variant="secondary"
               onClick={onClose}
-              className="rounded-lg border border-border px-4 py-2 text-sm text-muted-foreground hover:bg-accent hover:text-foreground"
             >
               Cancel
-            </button>
-            <button
+            </Button>
+            <Button
               type="submit"
-              disabled={isSubmitting || !name.trim()}
-              className="bg-gradient-brand flex items-center gap-2 rounded-lg px-4 py-2 text-sm font-medium text-white shadow-lg shadow-primary/20 hover:opacity-90 disabled:opacity-40 disabled:cursor-not-allowed"
+              disabled={!name.trim()}
+              loading={isSubmitting}
             >
-              {isSubmitting ? 'Forking…' : 'Fork Flow'}
-            </button>
+              Fork Flow
+            </Button>
           </div>
         </form>
       </div>