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/library/ImportFlowModal.tsx

@@ -7,6 +7,7 @@ import { toast } from '@/lib/toast'
 import { cn } from '@/lib/utils'
 import { useNavigate } from 'react-router-dom'
 import { getTreeEditorPath } from '@/lib/routing'
+import { Button } from '@/components/ui/Button'
 
 interface ImportFlowModalProps {
   onClose: () => void
@@ -227,28 +228,26 @@ export function ImportFlowModal({ onClose }: ImportFlowModalProps) {
         {/* Footer */}
         <div className="flex justify-end gap-2 border-t border-border px-5 py-3">
           {step === 'preview' && (
-            <button
+            <Button
+              variant="secondary"
               onClick={() => { setStep('pick'); setParsed(null); setParseError(null) }}
-              className="mr-auto rounded-lg border border-border px-3 py-2 text-sm text-muted-foreground hover:bg-accent hover:text-foreground"
+              className="mr-auto"
             >
               Back
-            </button>
+            </Button>
           )}
-          <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>
           {step === 'preview' && (
-            <button
+            <Button
               onClick={handleImport}
-              disabled={isImporting || !nameOverride.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={!nameOverride.trim()}
+              loading={isImporting}
             >
               <FileUp className="h-4 w-4" />
-              {isImporting ? 'Importing…' : 'Import as Draft'}
-            </button>
+              Import as Draft
+            </Button>
           )}
         </div>
       </div>