fix(availability): let editing availability re-sync with your calendar

[ianptkcs]

Summary

• Editing an already-submitted availability response never re-ran the Google Calendar auto-fill merge — only responding for the first time did. setAvailabilityAutomatically() was gated behind !userHasResponded in three places (Event.vue:777, Event.vue:1149, ScheduleOverlap.vue:4396), so calendar events added after your first response never showed up in the availability grid unless you deleted your response and recreated it from scratch.
• Added a "Sync from calendar" button, right below the existing calendar account list in the edit sidebar, that lets you explicitly re-run setAvailabilityAutomatically() while editing.
• Went with an explicit action instead of silently re-running the merge on every edit-open: unlike the Availability Groups flow, individual responses don't currently track which slots were manually touched, so auto-syncing on open could clobber manual overrides. An explicit button gives the same one-click refresh benefit as delete-and-recreate, without the destructive step.

Closes the reported issue: having to delete and re-add availability just to get it to reflect current Google Calendar events.

Demo

recording_2026-09-11_14-53-06.mp4

Test plan

• Respond to a normal (non-group) event with calendar auto-fill enabled
• Add a new event to your Google Calendar
• Reopen "Edit availability" — confirm the new event is not reflected yet (pre-existing, unchanged behavior)
• Click "Sync from calendar" — confirm it now reflects immediately, without deleting/recreating the response
• Manually toggle a slot, then confirm "Sync from calendar" only runs on click (doesn't silently fire and overwrite your edit)