refactor: enforce shared Modal component (#100)

* refactor: enforce shared Modal component in remaining custom modals

- ShareSessionModal: replaced custom modal markup with <Modal>
- CreateCategoryModal: replaced custom modal markup with <Modal>
- EditCategoryModal: replaced custom modal markup with <Modal>
- All now get focus trapping, Escape close, body scroll lock for free

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

* 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>

* refactor: adopt shared Button component in 20 page/component files

Replace raw <button> elements with <Button> across pages and remaining
components. 38 total files now use the shared Button component consistently.

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/StepLibraryPage.tsx

@@ -1,5 +1,6 @@
 import { useState } from 'react'
 import { Bookmark, Trash2 } from 'lucide-react'
+import { Button } from '@/components/ui/Button'
 import { useAuthStore } from '@/store/authStore'
 import { usePermissions } from '@/hooks/usePermissions'
 import { stepsApi } from '@/api/steps'
@@ -99,12 +100,9 @@ export default function StepLibraryPage() {
           </div>
         </div>
         {canCreateSteps && (
-          <button
-            onClick={() => setCreateOpen(true)}
-            className="rounded-md bg-gradient-brand px-4 py-2 text-sm font-medium text-white shadow-lg shadow-primary/20 hover:opacity-90"
-          >
+          <Button onClick={() => setCreateOpen(true)}>
             + Create Step
-          </button>
+          </Button>
         )}
       </div>
 
@@ -147,20 +145,22 @@ export default function StepLibraryPage() {
               <p className="mb-4 text-sm text-red-400">{deleteError}</p>
             )}
             <div className="flex gap-2">
-              <button
+              <Button
+                variant="secondary"
                 onClick={() => { setDeletingStep(null); setDeleteError(null) }}
                 disabled={isDeleting}
-                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 disabled:opacity-50"
+                className="flex-1"
               >
                 Cancel
-              </button>
-              <button
+              </Button>
+              <Button
+                variant="destructive"
                 onClick={handleDeleteConfirm}
-                disabled={isDeleting}
-                className="flex-1 rounded-md bg-red-500 px-4 py-2 text-sm font-medium text-white hover:bg-red-600 disabled:opacity-50"
+                loading={isDeleting}
+                className="flex-1"
               >
                 {isDeleting ? 'Deleting...' : 'Delete'}
-              </button>
+              </Button>
             </div>
           </div>
         </div>