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/admin/InviteCodesPage.tsx

@@ -1,5 +1,6 @@
 import { useState, useEffect, useCallback } from 'react'
 import { Plus, Copy, Trash2, Ticket, Mail, MailCheck, RefreshCw } from 'lucide-react'
+import { Button } from '@/components/ui/Button'
 import { DataTable, PageHeader, StatusBadge, ActionMenu, EmptyState } from '@/components/admin'
 import type { Column } from '@/components/admin'
 import { Modal } from '@/components/common/Modal'
@@ -215,16 +216,10 @@ export function InviteCodesPage() {
         title="Invite Codes"
         description="Create and manage registration invite codes with plan assignment"
         action={
-          <button
-            onClick={() => setCreateOpen(true)}
-            className={cn(
-              'flex items-center gap-2 rounded-md px-4 py-2 text-sm font-medium',
-              'bg-gradient-brand text-white shadow-lg shadow-primary/20 hover:opacity-90'
-            )}
-          >
+          <Button onClick={() => setCreateOpen(true)}>
             <Plus className="h-4 w-4" />
             Create Code
-          </button>
+          </Button>
         }
       />
 
@@ -249,19 +244,10 @@ export function InviteCodesPage() {
         size="sm"
         footer={
           <div className="flex justify-end gap-3">
-            <button
-              onClick={() => { setCreateOpen(false); resetForm() }}
-              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
-              onClick={handleCreate}
-              disabled={creating}
-              className="rounded-md bg-gradient-brand text-white shadow-lg shadow-primary/20 px-4 py-2 text-sm font-medium hover:opacity-90 disabled:opacity-50"
-            >
+            <Button variant="secondary" onClick={() => { setCreateOpen(false); resetForm() }}>Cancel</Button>
+            <Button onClick={handleCreate} loading={creating}>
               {creating ? 'Creating...' : 'Create'}
-            </button>
+            </Button>
           </div>
         }
       >