fix(vscode): default to the VS Code display language before a saved locale

The splash and the UI bundle both started in English on a fresh install
even when VS Code ran in a supported language, because only OpenChamber's
own saved locale was consulted. The HTML now exposes VS Code's display
language; the splash and detectInitialLocale use it until the user picks
a locale, which still wins.

Claude-Session: https://claude.ai/code/session_01VqV56Hez25hTxXH4ipJfzH

packages/ui/src/lib/i18n/runtime.ts

@@ -102,11 +102,23 @@ export function writeStoredLocale(locale: Locale): void {
   }
 }
 
+declare global {
+  interface Window {
+    /** The host application's display language (VS Code sets it), used before the user picks a locale. */
+    __OPENCHAMBER_HOST_LANGUAGE__?: string;
+  }
+}
+
 export function detectInitialLocale(): Locale {
   const stored = readStoredLocale();
   if (stored) {
     return stored;
   }
 
+  const hostLanguage = globalThis.window?.__OPENCHAMBER_HOST_LANGUAGE__;
+  if (hostLanguage) {
+    return normalizeLocale(hostLanguage);
+  }
+
   return DEFAULT_LOCALE;
 }