feat: custom menu pages with iframe embedding and CSP injection

Add configurable custom menu items that appear in sidebar, each rendering
an iframe-embedded external page. Includes shared URL builder with
src_host/src_url tracking, CSP frame-src multi-origin deduplication,
admin settings UI, and i18n support.

chore: bump version to 0.1.87.19

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

frontend/src/api/admin/settings.ts

@@ -4,6 +4,7 @@
  */
 
 import { apiClient } from '../client'
+import type { CustomMenuItem } from '@/types'
 
 export interface DefaultSubscriptionSetting {
   group_id: number
@@ -38,6 +39,7 @@ export interface SystemSettings {
   purchase_subscription_enabled: boolean
   purchase_subscription_url: string
   sora_client_enabled: boolean
+  custom_menu_items: CustomMenuItem[]
   // SMTP settings
   smtp_host: string
   smtp_port: number
@@ -99,6 +101,7 @@ export interface UpdateSettingsRequest {
   purchase_subscription_enabled?: boolean
   purchase_subscription_url?: string
   sora_client_enabled?: boolean
+  custom_menu_items?: CustomMenuItem[]
   smtp_host?: string
   smtp_port?: number
   smtp_username?: string