merge #69: probe embed signals via GET fallback when HEAD omits them (trinhthiminh650-tech)

# Conflicts:
#	src/client/builtins/tabs.tsx
#	src/client/locales.ts
#	src/client/prefs.ts
#	src/config.ts
#	src/index.ts
#	src/prefs-shared.ts

src/config.ts

@@ -140,6 +140,7 @@ export const PrefsSchema: z<SidebarPrefs> = z.object({
   browserInterceptLinks: z.boolean().default(true),
   browserInterceptHttp: z.boolean().default(true),
   browserInterceptHttps: z.boolean().default(false),
+  browserAllowedLoopback: z.string().default(''),
   // Per-feature enable switches are OPEN maps (any tab/viewer id, built-in or
   // external): an absent key means enabled, so old documents resolve to {}
   // (everything on) with no migration. Non-boolean values fail validation.