fix: re-enable CI lint job and fix ~65 lint errors (partial)

Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.

network/crypt_conn.go

@@ -132,7 +132,10 @@ func (cc *CryptConn) SendPacket(data []byte) error {
 		return err
 	}
 
-	cc.conn.Write(append(headerBytes, encData...))
+	_, err = cc.conn.Write(append(headerBytes, encData...))
+	if err != nil {
+		return err
+	}
 	cc.sentPackets++
 	cc.prevSendPacketCombinedCheck = combinedCheck