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>

frontend/src/components/procedural/IntakeFormModal.tsx

@@ -2,6 +2,7 @@ import { useState } from 'react'
 import type { IntakeFormField } from '@/types'
 import { PasswordInput } from '@/components/common/PasswordInput'
 import { cn } from '@/lib/utils'
+import { Button } from '@/components/ui/Button'
 
 interface IntakeFormModalProps {
   isOpen: boolean
@@ -240,19 +241,16 @@ export function IntakeFormModal({ isOpen, fields, treeName, onSubmit, onCancel }
 
           {/* Footer */}
           <div className="flex items-center justify-end gap-2 border-t border-border px-6 py-4">
-            <button
+            <Button
               type="button"
+              variant="secondary"
               onClick={onCancel}
-              className="rounded-md border border-border px-4 py-2 text-sm text-muted-foreground hover:bg-accent hover:text-foreground"
             >
               Cancel
-            </button>
-            <button
-              type="submit"
-              className="rounded-md bg-gradient-brand px-4 py-2 text-sm font-medium text-white shadow-lg shadow-primary/20 hover:opacity-90"
-            >
+            </Button>
+            <Button type="submit">
               Start Procedure
-            </button>
+            </Button>
           </div>
         </form>
       </div>