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.

apps/webui/src/messages/de.json

@@ -933,6 +933,11 @@
       "rolePingId": "Zu erwähnende Rolle",
       "cron": "Cron-Ausdruck",
       "runAt": "Ausführen am",
+      "timingMode": "Zeitplan",
+      "timingModes": {
+        "once": "Einmalig",
+        "cron": "Wiederkehrend (Cron)"
+      },
       "content": "Nachrichtentext",
       "embed": "Embed (optional)",
       "components": "Components-V2-Layout",
@@ -943,10 +948,15 @@
         "components_v2": "Components V2"
       },
       "componentsPreview": "(Components V2)",
-      "cronHint": "Cron-Ausdruck für wiederkehrende Nachrichten ODER ein einmaliges Datum/Uhrzeit — nicht beides. Wähle Text, Embed oder Components V2 (gegenseitig ausgeschlossen).",
+      "cronHint": "Einmalige Zeit oder Cron-Wiederholung — bezogen auf die Server-Zeitzone unter Allgemeine Einstellungen. Wähle Text, Embed oder Components V2 (gegenseitig ausgeschlossen).",
       "create": "Nachricht planen",
       "created": "Nachricht geplant.",
       "formIncomplete": "Bitte Kanal, Nachrichtenformat mit Inhalt sowie Cron-Ausdruck oder Datum/Uhrzeit ausfüllen.",
+      "errors": {
+        "invalidCron": "Ungültiger Cron-Ausdruck (5–6 Felder, z. B. 0 9 * * *).",
+        "invalidWhen": "Ungültiges Datum/Uhrzeit.",
+        "missingScheduleTime": "Bitte Cron-Ausdruck oder Datum/Uhrzeit angeben."
+      },
       "confirmDelete": "Diesen Zeitplan wirklich löschen?",
       "listTitle": "Geplante Nachrichten",
       "listDescription": "Anstehende und wiederkehrende geplante Nachrichten.",

apps/webui/src/messages/en.json

@@ -933,6 +933,11 @@
       "rolePingId": "Role to ping",
       "cron": "Cron expression",
       "runAt": "Run at",
+      "timingMode": "Schedule",
+      "timingModes": {
+        "once": "One-time",
+        "cron": "Recurring (cron)"
+      },
       "content": "Message content",
       "embed": "Embed (optional)",
       "components": "Components V2 layout",
@@ -943,10 +948,15 @@
         "components_v2": "Components V2"
       },
       "componentsPreview": "(Components V2)",
-      "cronHint": "Set a cron expression for recurring messages, or a one-time date/time — not both. Choose text, embed, or Components V2 (mutually exclusive).",
+      "cronHint": "One-time time or cron recurrence — evaluated in the server timezone under General Settings. Choose text, embed, or Components V2 (mutually exclusive).",
       "create": "Schedule message",
       "created": "Message scheduled.",
       "formIncomplete": "Please fill channel, choose a message format with content, and either a cron expression or a date/time.",
+      "errors": {
+        "invalidCron": "Invalid cron expression (5–6 fields, e.g. 0 9 * * *).",
+        "invalidWhen": "Invalid date/time.",
+        "missingScheduleTime": "Provide a cron expression or a date/time."
+      },
       "confirmDelete": "Really delete this schedule?",
       "listTitle": "Scheduled messages",
       "listDescription": "Upcoming and recurring scheduled messages.",