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/repo_interfaces.go

@@ -159,6 +159,13 @@ type UserRepo interface {
 	SetPasswordByDiscordID(discordID string, hash []byte) error
 	GetByIDAndUsername(charID uint32) (userID uint32, username string, err error)
 	BanUser(userID uint32, expires *time.Time) error
+	// GetLanguage returns the user's preferred language code (e.g. "en", "jp").
+	// An empty string means the user has no preference set and the server
+	// default should be used.
+	GetLanguage(userID uint32) (string, error)
+	// SetLanguage stores the user's preferred language code. Passing an empty
+	// string clears the preference (reverts to server default on next login).
+	SetLanguage(userID uint32, lang string) error
 }
 
 // GachaRepo defines the contract for gacha system data access.