store: remove stale calendar events after date changes

[anish749]

Cross-file dedup pass for calendar events in Maintain.

When an event moves dates or a multi-day event shrinks, stale copies
linger in old date files. Scans all date files per calendar, picks the
most recently updated version of each event, and removes copies from
files outside the event's current DatesForStorage range.

[anish749]

Two issues to address:

1. Path construction belongs in paths module. filepath.Join(ad.Path(), paths.GcalendarSubdir) in deduplicateCalendarEvents should be a method on AccountDir (e.g. ad.GcalendarDir() returning the path), keeping the store layer free of path construction. Same layering rule as the rest of the codebase.

2. "Latest" event version uses string comparison on Runtime.Updated. This works for RFC 3339 timestamps lexicographically, but it's implicit. Consider parsing to time.Time for the comparison, or at minimum add a comment explaining why string comparison is safe here. Also need to handle the case where Updated is empty.

[anish749]

Superseded by #266 which takes a cleaner reshuffle approach.