fix: address code review findings (critical + high + medium)

Critical:
- retryOnBusy: return lastErr after exhausted retries instead of nil

High:
- Remove debug console.log from use-paired-devices.ts

Medium:
- ChannelGeneralTab: sync state from instance prop via useEffect
- i18n: add MsgCannotRemoveLastWriter key + translations (en/vi/zh)
- use-channel-crud: refetch after delete instead of optimistic remove

internal/http/channel_instances.go

@@ -437,7 +437,7 @@ func (h *ChannelInstancesHandler) handleRemoveWriter(w http.ResponseWriter, r *h
 		}
 	}
 	if allowCount <= 1 {
-		writeError(w, http.StatusConflict, protocol.ErrFailedPrecondition, "cannot remove the last file writer")
+		writeError(w, http.StatusConflict, protocol.ErrFailedPrecondition, i18n.T(locale, i18n.MsgCannotRemoveLastWriter))
 		return
 	}
 	if err := h.configPermStore.Revoke(r.Context(), agentID, groupID, "file_writer", userID); err != nil {