Add plain text clipboard transfers and refine input feedback

.env.example

@@ -25,6 +25,7 @@ SPACE_INVITE_CODE_LENGTH=4
 SPACE_MAX_DEVICES=10
 OFFLINE_GRACE_MS=3000
 MAX_URL_PAYLOAD_SIZE_KB=1024
+MAX_TEXT_PAYLOAD_SIZE_KB=256
 MAX_ITEMS_PER_TRANSFER=20
 DEFAULT_URL_RETENTION_DAYS=7
 GUEST_REQUESTS_PER_MINUTE=5