fix(webapp): clearer login error when WHITELISTED_EMAILS rejects an address

Author: samejr

.server-changes/clearer-whitelist-login-error.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: improvement
+---
+
+Clearer login error when an email address is blocked by the WHITELISTED_EMAILS setting: the message now explains the address isn't allowed on this instance instead of the ambiguous "This email is unauthorized".

apps/webapp/app/utils/email.ts

@@ -7,6 +7,8 @@ export function assertEmailAllowed(email: string) {
   }
 
   if (!emailMatchesPattern(env.WHITELISTED_EMAILS, email)) {
-    throw new Error("This email is unauthorized");
+    // Surfaced verbatim on the login page. Name the actual policy so a
+    // rejection on a restricted instance reads as configuration, not a bug.
+    throw new Error("This email address isn't allowed to sign in on this instance.");
   }
 }