EditEventDialog "Time in UTC" caption is blank on open and stale after "Now" #348

Both incident/EditEventDialog.vue and case/EditEventDialog.vue render
Time in UTC is {{ formatToUTC(started_at_in_utc) }}, but started_at_in_utc
is only ever written by update_started_at(), which fires solely on the
picker's update:model-value emit.

Two consequences, in both copies:

• Blank on open. mounted() sets local_started_at = this.started_at but
  never seeds started_at_in_utc, so it stays "" and formatToUTC("")
  returns undefined. The caption reads "Time in UTC is " until the user opens
  the picker and confirms a time.
• Stale after "Now". setTimeToNow() assigns local_started_at directly.
  That drives the picker through its modelValue watcher but emits nothing, so
  the caption keeps showing the previously picked value while the text field
  above it shows the new one.

Pre-existing and shared by both dialogs; unrelated to the eventStart defect
fixed in #347, which deliberately left this alone to keep that diff to one line.