Fix stray calendar cells

[marcorieser]

Closes: #186

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Note

Low Risk
Display-only calendar grouping fix with added tests; no auth, persistence, or API contract changes.

Overview
Stops the events calendar tag from appending extra days when occurrences straddle the grid in a non-app timezone.

calendar() now builds the month grid in the requested timezone and keeps only occurrence groups whose dates already exist in that grid, so merge cannot add cells before the first week or after the last. Tests cover behind-UTC timezones, overnight spans past the last day, and DST.

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