Enhance scheduler functionality with timezone support and error handling

- Updated scheduling logic to utilize the guild's timezone for cron expressions and one-time scheduling.
- Introduced a new `getGuildTimezone` function to retrieve the timezone from guild settings.
- Enhanced error handling for scheduling inputs, providing localized error messages for invalid cron expressions and missing schedule times.
- Improved the WebUI to support a new timing mode selection (once vs. cron) and updated input fields accordingly.
- Added localization entries for new scheduling-related messages in both English and German.
- Refactored job scheduling and removal logic to ensure proper handling of scheduled jobs in the queue.

docs/PHASE-TRACKING.md

@@ -598,3 +598,20 @@
 - [ ] Online-Kanal zeigt sinnvolle Zahl (nicht dauerhaft 0)
 - [ ] Bot ohne „Kanäle verwalten“ → Config gespeichert; Bot-Log warnt; nach Permission-Fix erneut speichern
 
+## Post-Phase – Scheduler Cron / Run-at Fix (Status: implementiert)
+
+### Abgeschlossen (Code)
+
+- Cron und einmaliges „Ausführen am“ nutzen die Guild-Zeitzone (`GuildSettings.timezone`)
+- Cron über BullMQ `upsertJobScheduler` inkl. `tz`; Delete entfernt Job-Scheduler korrekt
+- Dashboard: Timing-Mode (einmalig vs. Cron), naive `datetime-local` an Server, Fehlertexte i18n
+- Dark Mode: `color-scheme` + Input `text-foreground` für datetime-local Lesbarkeit
+- Shared Helper `resolveScheduleRunAt` / `guildLocalDateTimeToUtc`
+
+### Manuell testen
+
+- [ ] WebUI einmalig: Datum/Uhrzeit in Server-Zeitzone → Nachricht zur erwarteten lokalen Zeit
+- [ ] WebUI Cron `0 9 * * *` bei Europe/Berlin → ca. 09:00 Berlin (nicht UTC)
+- [ ] Cron-Plan löschen → keine weiteren Sends
+- [ ] Dark Mode: Cron-Feld und datetime-local lesbar/bedienbar
+