⚡️(frontend) backport phone number formatting to the backend

Use the Python port of Google's phone number library on the backend
instead of the JS one on the client. Saves 140Kb of unnecessary JS
and avoids a complex dynamic import that would have been required to
optimize loading.

The transformation is static: the phone number lives in the Django
settings, so the backend can format it once and pass the result to
the client. This avoids every client loading the 140Kb library and
re-computing the same information.

Moreover, within a single client, the transformation was previously
re-computed several times during a webapp lifecycle.

src/backend/core/api/__init__.py

@@ -8,6 +8,8 @@ from rest_framework import views as drf_views
 from rest_framework.decorators import api_view
 from rest_framework.response import Response
 
+from core.utils import build_telephony_config
+
 
 def exception_handler(exc, context):
     """Handle Django ValidationError as an accepted exception.
@@ -58,13 +60,7 @@ def get_frontend_configuration(request):
                 "allowed_mimetypes"
             ],
         },
-        "telephony": {
-            "enabled": settings.ROOM_TELEPHONY_ENABLED,
-            "phone_number": settings.ROOM_TELEPHONY_PHONE_NUMBER
-            if settings.ROOM_TELEPHONY_ENABLED
-            else None,
-            "default_country": settings.ROOM_TELEPHONY_DEFAULT_COUNTRY,
-        },
+        "telephony": build_telephony_config(),
         "subtitle": {"enabled": settings.ROOM_SUBTITLE_ENABLED},
         "livekit": {
             "url": settings.LIVEKIT_CONFIGURATION["url"],