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/components/tree-editor/NodeEditorModal.tsx

@@ -5,6 +5,7 @@ import { NodeFormDecision } from './NodeFormDecision'
 import { NodeFormAction } from './NodeFormAction'
 import { NodeFormResolution } from './NodeFormResolution'
 import type { TreeStructure } from '@/types'
+import { Button } from '@/components/ui/Button'
 
 interface NodeEditorModalProps {
   node: TreeStructure
@@ -65,20 +66,19 @@ export function NodeEditorModal({ node, onClose, isNewNode = false }: NodeEditor
 
   const footerContent = (
     <div className="flex justify-end gap-2">
-      <button
+      <Button
         type="button"
+        variant="secondary"
         onClick={handleCancel}
-        className="rounded-md border border-border px-4 py-2 text-sm font-medium text-muted-foreground hover:bg-accent hover:text-foreground"
       >
         Cancel
-      </button>
-      <button
+      </Button>
+      <Button
         type="button"
         onClick={handleSave}
-        className="rounded-md bg-gradient-brand text-white shadow-lg shadow-primary/20 px-4 py-2 text-sm font-medium hover:opacity-90"
       >
         Done
-      </button>
+      </Button>
     </div>
   )