- Add I18nService with per-user language preferences (de/en)
- Add !language/!sprache command to all 4 bots (todo, calendar, contacts, clock)
- Add fallback behavior: messages without commands create tasks/events/contacts/timers
- Improve clock bot duration parsing to accept bare numbers as minutes (e.g. "25" = 25min)
- Add support for more duration formats: "25 minuten", "1 stunde", etc.
Language preferences stored in SessionService, default configurable via BOT_DEFAULT_LANGUAGE env var.