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.

server/channelserver/handlers_register_test.go

@@ -4,7 +4,6 @@ import (
 	"testing"
 
 	"erupe-ce/common/byteframe"
-	"erupe-ce/network/mhfpacket"
 )
 
 // createMockServerWithRaviente creates a mock server with raviente and semaphore
@@ -26,9 +25,6 @@ func TestRavienteInitialization(t *testing.T) {
 		state:    make([]uint32, 30),
 		support:  make([]uint32, 30),
 	}
-	if r == nil {
-		t.Fatal("Raviente is nil")
-	}
 	if len(r.register) != 30 {
 		t.Errorf("register length = %d, want 30", len(r.register))
 	}
@@ -221,9 +217,4 @@ func TestMockServerPacketHandling(t *testing.T) {
 		t.Errorf("ByteFrame length = %d, want 4", len(bf.Data()))
 	}
 
-	// Verify packet types can be instantiated
-	pkt := &mhfpacket.MsgSysAck{}
-	if pkt == nil {
-		t.Error("Failed to create MsgSysAck")
-	}
 }