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/stores/app.ts

@@ -327,6 +327,7 @@ export const useAppStore = defineStore('app', () => {
         hide_ccs_import_button: false,
         purchase_subscription_enabled: false,
         purchase_subscription_url: '',
+        custom_menu_items: [],
         linuxdo_oauth_enabled: false,
         sora_client_enabled: false,
         version: siteVersion.value