fix: custom menu security hardening and code quality improvements

- Add admin menu permission check in CustomPageView (visibility + role)
- Sanitize SVG content with DOMPurify before v-html rendering (XSS prevention)
- Decouple router.go from dto package using anonymous struct
- Consolidate duplicate parseCustomMenuItems into dto.ParseCustomMenuItems
- Enhance menu item validation (count, length, ID uniqueness limits)
- Add audit logging for purchase_subscription and custom_menu_items changes
- Update API contract test to include custom_menu_items field

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/utils/sanitize.ts

@@ -0,0 +1,6 @@
+import DOMPurify from 'dompurify'
+
+export function sanitizeSvg(svg: string): string {
+  if (!svg) return ''
+  return DOMPurify.sanitize(svg, { USE_PROFILES: { svg: true, svgFilters: true } })
+}