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_mocks_test.go

@@ -672,6 +672,8 @@ func (m *mockUserRepoForItems) GetByIDAndUsername(_ uint32) (uint32, string, err
 	return 0, "", nil
 }
 func (m *mockUserRepoForItems) BanUser(_ uint32, _ *time.Time) error { return nil }
+func (m *mockUserRepoForItems) GetLanguage(_ uint32) (string, error) { return "", nil }
+func (m *mockUserRepoForItems) SetLanguage(_ uint32, _ string) error { return nil }
 
 // --- mockStampRepoForItems ---