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/sys_session.go

@@ -48,6 +48,7 @@ type Session struct {
 	prevGuildID      uint32 // Stores the last GuildID used in InfoGuild
 	charID           uint32
 	userID           uint32
+	clientLang       string // Per-session language preference; empty = use server default
 	logKey           []byte
 	sessionStart     int64
 	courses          []mhfcourse.Course
@@ -109,6 +110,28 @@ func NewSession(server *Server, conn net.Conn) *Session {
 	return s
 }
 
+// Lang returns the session's effective language code, falling back to the
+// server's globally configured language when no per-user preference has been
+// loaded. Callers should use this instead of reading erupeConfig.Language
+// directly so that later phases can route localized content per session.
+func (s *Session) Lang() string {
+	s.Lock()
+	lang := s.clientLang
+	s.Unlock()
+	if lang != "" {
+		return lang
+	}
+	return s.server.erupeConfig.Language
+}
+
+// SetLang updates the session's in-memory language preference. Persistence
+// to the database is the caller's responsibility (via userRepo.SetLanguage).
+func (s *Session) SetLang(lang string) {
+	s.Lock()
+	s.clientLang = lang
+	s.Unlock()
+}
+
 // Start starts the session packet send and recv loop(s).
 func (s *Session) Start() {
 	s.logger.Debug("New connection", zap.String("RemoteAddr", s.rawConn.RemoteAddr().String()))