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/types/index.ts

@@ -75,6 +75,15 @@ export interface SendVerifyCodeResponse {
   countdown: number
 }
 
+export interface CustomMenuItem {
+  id: string
+  label: string
+  icon_svg: string
+  url: string
+  visibility: 'user' | 'admin'
+  sort_order: number
+}
+
 export interface PublicSettings {
   registration_enabled: boolean
   email_verify_enabled: boolean
@@ -93,6 +102,7 @@ export interface PublicSettings {
   hide_ccs_import_button: boolean
   purchase_subscription_enabled: boolean
   purchase_subscription_url: string
+  custom_menu_items: CustomMenuItem[]
   linuxdo_oauth_enabled: boolean
   sora_client_enabled: boolean
   version: string