fix: resolve all remaining lint errors (errcheck) across 49 files

Fix unchecked error returns on bf.Seek(), db.Exec(), QueryRow().Scan(),
pkt.Build(), logger.Sync(), and binary.Write() calls. The linter now
passes with 0 errors, build compiles, and all tests pass with -race.

network/mhfpacket/msg_sys_packets_test.go

@@ -31,7 +31,7 @@ func TestMsgSysCastBinaryParse(t *testing.T) {
 			bf.WriteUint8(tt.messageType)
 			bf.WriteUint16(uint16(len(tt.payload)))
 			bf.WriteBytes(tt.payload)
-			bf.Seek(0, io.SeekStart)
+			_, _ = bf.Seek(0, io.SeekStart)
 
 			pkt := &MsgSysCastBinary{}
 			err := pkt.Parse(bf, &clientctx.ClientContext{})