fix: replace panic calls with proper error handling

Remove 51 panic() calls from handler code and replace with:
- Proper error logging using zap
- Appropriate client error responses (doAckBufFail, doAckSimpleFail)
- Graceful error recovery instead of server crashes

Files updated:
- handlers_guild_scout.go (9 panics)
- handlers_guild_tresure.go (10 panics)
- handlers_guild.go (7 panics + dead code removal)
- handlers_mail.go (5 panics)
- handlers.go (9 panics)
- handlers_tower.go (2 panics)
- handlers_clients.go (3 panics)
- handlers_guild_alliance.go (1 panic)
- handlers_quest.go (1 panic)
- handlers_rengoku.go (1 panic)
- handlers_stage.go (1 panic)
- handlers_data.go (1 panic)
- handlers_cafe.go (1 panic)
- signserver/sign_server.go (1 panic)

Remaining panics (3) are in test files and compression library
where panicking on programming errors is appropriate.

server/channelserver/handlers_data.go

@@ -345,7 +345,9 @@ func handleMsgMhfGetPaperData(s *Session, p mhfpacket.MHFPacket) {
 		s.logger.Info("GET_PAPER request for unknown type")
 	}
 	if err != nil {
-		panic(err)
+		s.logger.Error("failed to decode paper hex data", zap.Error(err))
+		doAckBufSucceed(s, pkt.AckHandle, []byte{0x00, 0x00, 0x00, 0x00})
+		return
 	}
 	doAckBufSucceed(s, pkt.AckHandle, data)
 }