Fix: Add default_locale to manifest for Chrome extension compatibility

Added 'default_locale': 'en' to manifest.json to resolve Chrome error
when loading extension with _locales directory.

Error was: 'Lokalisierung wurde verwendet, in der Manifest-Datei war
jedoch kein Wert für default_locale angegeben'

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

public/manifest.json

@@ -3,6 +3,7 @@
   "name": "BinectChrome",
   "version": "1.0.0",
   "description": "Send PDFs from cloud applications directly to Binect for physical mail delivery",
+  "default_locale": "en",
   "permissions": [
     "downloads",
     "storage"