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>

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

frontend/src/components/library/ExportFlowModal.tsx

@@ -2,6 +2,7 @@ import { useState, useEffect } from 'react'
 import { Download, X } from 'lucide-react'
 import { flowTransferApi } from '@/api/flowTransfer'
 import { toast } from '@/lib/toast'
+import { Button } from '@/components/ui/Button'
 
 interface ExportFlowModalProps {
   treeId: string
@@ -79,20 +80,16 @@ export function ExportFlowModal({ treeId, treeName, onClose }: ExportFlowModalPr
 
         {/* Footer */}
         <div className="flex justify-end gap-2 border-t border-border px-5 py-3">
-          <button
-            onClick={onClose}
-            className="rounded-lg border border-border px-4 py-2 text-sm text-muted-foreground hover:bg-accent hover:text-foreground"
-          >
+          <Button variant="secondary" onClick={onClose}>
             Cancel
-          </button>
-          <button
+          </Button>
+          <Button
             onClick={handleExport}
-            disabled={isExporting}
-            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"
+            loading={isExporting}
           >
             <Download className="h-4 w-4" />
-            {isExporting ? 'Exporting…' : 'Download .rfflow'}
-          </button>
+            Download .rfflow
+          </Button>
         </div>
       </div>
     </div>