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/MySharesPage.tsx

@@ -1,6 +1,7 @@
 import { useState, useEffect, useCallback } from 'react'
 import { Link, useNavigate } from 'react-router-dom'
 import { Globe, Users, Copy, Check, Link2, ExternalLink, Trash2, ArrowLeft } from 'lucide-react'
+import { Button } from '@/components/ui/Button'
 import { Spinner } from '@/components/common/Spinner'
 import { EmptyState } from '@/components/common/EmptyState'
 import { ConfirmDialog } from '@/components/common/ConfirmDialog'
@@ -110,12 +111,9 @@ export default function MySharesPage() {
         <div className="bg-card border border-red-400/20 rounded-xl p-6">
           <div className="text-center">
             <p className="text-red-400 text-sm mb-4">{error}</p>
-            <button
-              onClick={fetchShares}
-              className="bg-gradient-brand text-white shadow-lg shadow-primary/20 hover:opacity-90 rounded-md px-4 py-2 text-sm font-medium transition-colors"
-            >
+            <Button onClick={fetchShares}>
               Try again
-            </button>
+            </Button>
           </div>
         </div>
       </div>
@@ -147,12 +145,9 @@ export default function MySharesPage() {
             title="No shared sessions"
             description="Share a session from the session detail page to create a link"
             action={
-              <button
-                onClick={() => navigate('/sessions')}
-                className="bg-gradient-brand text-white shadow-lg shadow-primary/20 hover:opacity-90 rounded-md px-4 py-2 text-sm font-medium transition-colors"
-              >
+              <Button onClick={() => navigate('/sessions')}>
                 Go to Sessions
-              </button>
+              </Button>
             }
           />
         </div>
@@ -201,14 +196,10 @@ export default function MySharesPage() {
 
                 {/* Actions */}
                 <div className="flex flex-wrap items-center gap-2">
-                  <button
+                  <Button
+                    size="sm"
                     onClick={() => handleCopyLink(share)}
-                    className={cn(
-                      'inline-flex items-center gap-1.5 rounded-md px-3 py-1.5 text-sm font-medium transition-colors',
-                      isCopied
-                        ? 'bg-emerald-400/10 text-emerald-400'
-                        : 'bg-gradient-brand text-white shadow-lg shadow-primary/20 hover:opacity-90'
-                    )}
+                    className={isCopied ? 'bg-emerald-400/10 text-emerald-400 shadow-none hover:opacity-100' : ''}
                   >
                     {isCopied ? (
                       <Check className="h-3.5 w-3.5" />
@@ -216,7 +207,7 @@ export default function MySharesPage() {
                       <Copy className="h-3.5 w-3.5" />
                     )}
                     {isCopied ? 'Copied' : 'Copy Link'}
-                  </button>
+                  </Button>
 
                   <Link
                     to={`/sessions/${share.session_id}`}
@@ -226,13 +217,14 @@ export default function MySharesPage() {
                     View Session
                   </Link>
 
-                  <button
+                  <Button
+                    variant="destructive"
+                    size="sm"
                     onClick={() => setRevokeTarget(share)}
-                    className="inline-flex items-center gap-1.5 text-red-400 hover:text-red-300 hover:bg-red-400/10 rounded-md px-3 py-1.5 text-sm transition-colors"
                   >
                     <Trash2 className="h-3.5 w-3.5" />
                     Revoke
-                  </button>
+                  </Button>
                 </div>
               </div>
             )