fix(guild): fix nil panics causing clan menu softlock (#171)

The crash was in handleMsgMhfGetGuildManageRight where a variable
shadowing bug (guild, err := instead of guild, err =) left the
outer guild pointer nil after the inner GetByID lookup succeeded,
panicking at guild.MemberCount. This is confirmed by the user's
stack trace pointing to handlers_guild.go:234.

Also fix 6 other nil-dereference risks across guild handlers:
- handleMsgMhfArrangeGuildMember: guild nil after GetByID
- handleMsgMhfEnumerateGuildMember: alliance nil when AllianceID > 0
- handleMsgMhfUpdateGuildIcon: guild and characterInfo nil
- handleMsgMhfOperateGuild: guild nil, characterGuildInfo nil
- handleAvoidLeadershipUpdate: characterGuildData nil

Improve panic recovery to log opcode and full stack trace so
future panics can be diagnosed from console screenshots.

server/channelserver/sys_session.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"runtime/debug"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -240,7 +241,12 @@ func (s *Session) handlePacketGroup(pktGroup []byte) {
 	// This shouldn't be needed, but it's better to recover and let the connection die than to panic the server.
 	defer func() {
 		if r := recover(); r != nil {
-			s.logger.Error("Recovered from panic", zap.String("name", s.Name), zap.Any("panic", r))
+			s.logger.Error("Recovered from panic",
+				zap.String("name", s.Name),
+				zap.Stringer("opcode", opcode),
+				zap.Any("panic", r),
+				zap.String("stack", string(debug.Stack())),
+			)
 		}
 	}()