fix(scheduled-tasks): preserve monthly anchor when reselecting Monthly

Selecting Monthly from the frequency dropdown hard-reset monthlyMode to
day-of-month, silently dropping a previously chosen nth-/last-weekday
anchor when switching cadence away and back. Preserve the existing mode
on reselect, mirroring how the last recurring cadence is restored across
the recurring toggle.

Author: waleedlatif1

apps/sim/app/workspace/[workspaceId]/scheduled-tasks/components/task-modal/recurrence-section.tsx

@@ -154,7 +154,7 @@ export function RecurrenceSection({ recurrence, onChange, launchDate }: Recurren
           ...recurrence,
           frequency: 'monthly',
           weekdays: [],
-          monthlyMode: 'day-of-month',
+          monthlyMode: recurrence.monthlyMode ?? 'day-of-month',
           cron: undefined,
         })
         return