Simplify connection flow, mobile transfer UI, and header actions

.env.example

@@ -20,6 +20,8 @@ GUEST_SESSION_RETENTION_DAYS=30
 LOCAL_REQUEST_TIMEOUT_SECONDS=60
 ACCEPTED_UPLOAD_START_HOURS=24
 SPACE_RETENTION_MINUTES=10
+# Invite codes can be 4 or 6 characters.
+SPACE_INVITE_CODE_LENGTH=4
 SPACE_MAX_DEVICES=10
 OFFLINE_GRACE_MS=3000
 MAX_URL_PAYLOAD_SIZE_KB=1024