fix: parse raw public translation responses in banner

The public translations endpoint returns the raw strings dictionary, but
the banner expected a wrapped { strings } response and therefore fell back
to English for Thai translations.

Update fetchTranslations() to accept the raw public API shape while keeping
compatibility with wrapped responses. Make default_language optional in the
banner SiteConfig type so old/mocked configs still typecheck.

apps/banner/src/i18n.ts

@@ -95,9 +95,10 @@ export async function fetchTranslations(
   try {
     const resp = await fetch(`${apiBase}/api/v1/translations/${siteId}/${locale}`);
     if (!resp.ok) return null;
-    // API returns { strings: { ... } }
-    const data = (await resp.json()) as { strings?: Partial<TranslationStrings> };
-    return data.strings ?? null;
+    // Public API returns the raw strings dict. Accept a wrapped `{ strings }`
+    // shape too for backwards compatibility with older mocks/clients.
+    const data = await resp.json() as Partial<TranslationStrings> | { strings?: Partial<TranslationStrings> };
+    return 'strings' in data && data.strings ? data.strings : data as Partial<TranslationStrings>;
   } catch {
     return null;
   }