🐛(frontend) fall back to user.full_name on request-entry

Since the username refactoring, the username in the store could be
undefined when the join input was pre-filled from user.full_name,
because no keystroke was needed to populate the store.

This led to a 400 error on the request-entry endpoint whenever the
user joined without editing the pre-filled name.

Fall back to user.full_name when the store username is missing, so
the endpoint always receives a value.

Acknowledged as a somewhat wobbly fix, but ships as-is until the
underlying flow is reworked.

CHANGELOG.md

@@ -32,6 +32,7 @@ and this project adheres to
 - 🐛(summary) properly detect when failure webhook should be sent
 - 🐛(backend) preserve recording metadata when updating room access
 - 🐛(backend) allow any string as sub in the API serializer
+- 🐛(frontend) fall back to user.full_name on request-entry
 
 
 ## [1.24.0] - 2026-07-21