feat(i18n): per-session language preference and !lang command

Phase A plumbing for #188. Adds a users.language column (migration
0022), UserRepo.GetLanguage/SetLanguage, and Session.Lang()/SetLang
accessors so future phases can resolve localized content per session
instead of falling back to the server-wide config.Language.

The preference is loaded from the DB on login and persisted via a new
!lang <en|jp|fr|es> chat command that shows the current language when
called without an argument, validates the code (case-insensitive), and
replies in the newly selected language so the switch is visible
immediately. An empty stored value falls back to config.Language.

sys_language.go exposes getLangStringsFor(code) as the new dispatch
primitive; getLangStrings(server) is now a thin wrapper so existing
callers keep working unchanged. isSupportedLang + supportedLangs keep
the !lang validator in sync with the dispatcher.

Localized quest/scenario content and per-session i18n lookups in
existing handlers are deliberately out of scope for phase A — this
commit ships only the plumbing so it can be reviewed and deployed
independently.

server/channelserver/handlers_session.go

@@ -76,6 +76,15 @@ func handleMsgSysLogin(s *Session, p mhfpacket.MHFPacket) {
 	}
 	s.userID = userID
 
+	// Load per-user language preference. A DB error or an empty value both
+	// mean "use the server default", which is what Session.Lang() returns
+	// when clientLang is empty — so we don't need to fail the login here.
+	if lang, langErr := s.server.userRepo.GetLanguage(userID); langErr == nil && lang != "" {
+		s.SetLang(lang)
+	} else if langErr != nil {
+		s.logger.Warn("Failed to load user language preference", zap.Error(langErr), zap.Uint32("userID", userID))
+	}
+
 	if s.captureConn != nil {
 		s.captureConn.SetSessionInfo(s.charID, s.userID)
 	}