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/session/SaveSessionAsTreeModal.tsx

@@ -1,6 +1,7 @@
 import { useState } from 'react'
 import { X } from 'lucide-react'
 import { cn } from '@/lib/utils'
+import { Button } from '@/components/ui/Button'
 
 interface SaveSessionAsTreeModalProps {
   isOpen: boolean
@@ -130,27 +131,20 @@ export function SaveSessionAsTreeModal({
 
           {/* Actions */}
           <div className="flex justify-end gap-2 pt-2">
-            <button
+            <Button
               type="button"
+              variant="secondary"
               onClick={onClose}
               disabled={isSaving}
-              className={cn(
-                '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'
-              )}
             >
               Cancel
-            </button>
-            <button
+            </Button>
+            <Button
               type="submit"
-              disabled={isSaving}
-              className={cn(
-                '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'
-              )}
+              loading={isSaving}
             >
-              {isSaving ? 'Saving...' : 'Save as Tree'}
-            </button>
+              Save as Tree
+            </Button>
           </div>
         </form>
       </div>