refactor: replace panic calls with structured error handling

Replace ~25 panic() calls in non-fatal code paths with proper
s.logger.Error + return patterns. Panics in handler code crashed
goroutines (caught by defer/recover but still disruptive) instead
of failing gracefully.

Key changes:
- SJISToUTF8 now returns (string, error); all 30+ callers updated
- Handler DB/IO panics replaced with log + return/ack fail
- Unhandled switch-case panics replaced with logger.Error
- Sign server Accept() panic replaced with log + continue
- Dead unreachable panic in guild_model.go removed
- deltacomp patch error logs and returns partial data

Panics intentionally kept: ByteFrame sentinel, unimplemented
packet stubs, os.Exit in main.go.

server/channelserver/guild_model.go

@@ -484,10 +484,6 @@ func CreateGuild(s *Session, guildName string) (int32, error) {
 		return 0, err
 	}
 
-	if err != nil {
-		panic(err)
-	}
-
 	guildResult, err := transaction.Query(
 		"INSERT INTO guilds (name, leader_id) VALUES ($1, $2) RETURNING id",
 		guildName, s.charID,