fix(lotus): show the SFU token service's refusal reason instead of "Something went wrong"

When the voice-limit guard (or any JWT service) answers 403 with a reason —
"This voice channel is full.", "You don't have permission to …" — EC wrapped it
in FailToGetOpenIdToken and the user saw the generic error page with
OPEN_ID_ERROR. A 403 MatrixError with a non-empty `error` now throws
SFUTokenRefusedError ("Can't join this call" + the server's sentence), so the
reason is the description. Other failures are unchanged. Unit-tested.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

locales/en/app.json

@@ -116,6 +116,7 @@
     "peer_connection_timeout_description": "Connection to the media server timed out. Try switching to a different network or disabling your VPN. If the problem persists, see our <0>troubleshooting guide</0> or contact your server administrator.",
     "room_creation_restricted": "Failed to create call",
     "room_creation_restricted_description": "Call creation might be restricted to authorized users only. Try again later, or contact your server admin if the problem persists.",
+    "sfu_token_refused": "Can't join this call",
     "sticky_events_required": "Homeserver does not support Matrix 2.0 calls",
     "sticky_events_required_description": "This deployment is configured to use Matrix 2.0 call mode, but the homeserver does not advertise support for sticky events (MSC4354). Ask your server admin to upgrade, or switch the deployment to a compatible mode.",
     "unexpected_ec_error": "An unexpected error occurred (<0>Error Code:</0> <1>{{ errorCode }}</1>). Please contact your server admin."