Pass the relay's work_email_required error through the viewer server

Classify the relay's 400 work_email_required separately from a malformed
address so the browser can show the "use your work email" message.

strix/viewer/auth.py

@@ -123,12 +123,16 @@ def _parse_body(raw: bytes) -> dict[str, Any]:
 
 def otp_start(email: str) -> None:
     """Ask the relay to email a verification code. Raises RelayError on failure."""
-    status, _ = _post_json("/api/oss/otp/start", {"email": email}, timeout=_OTP_TIMEOUT)
+    status, data = _post_json("/api/oss/otp/start", {"email": email}, timeout=_OTP_TIMEOUT)
     if status == 200:
         return
     if status == 429:
         raise RelayError("rate_limited")
     if status == 400:
+        # The relay uses 400 both for a malformed address and, separately, to
+        # reject a free/personal email domain (it wants a work email).
+        if data.get("error") == "work_email_required":
+            raise RelayError("work_email_required")
         raise RelayError("invalid_email")
     raise RelayError("unavailable")
 

strix/viewer/server.py

@@ -284,6 +284,7 @@ def _make_handler(state: _ViewerState) -> type[BaseHTTPRequestHandler]:
             status_by_code = {
                 "rate_limited": HTTPStatus.TOO_MANY_REQUESTS,
                 "invalid_email": HTTPStatus.BAD_REQUEST,
+                "work_email_required": HTTPStatus.BAD_REQUEST,
                 "invalid_code": HTTPStatus.FORBIDDEN,
                 "reverify": HTTPStatus.UNAUTHORIZED,
                 "forbidden": HTTPStatus.FORBIDDEN,