fix(calendar): support --reminder popup:0m

[salmonumbrella]

Summary

• force-send EventReminder.Minutes when value is 0 so Calendar API receives an explicit 0-minute override
• add regression test for buildReminders([]string{"popup:0m"})

Why

calendar create --reminder popup:0m currently fails with:
Google API error (400 required): Missing override reminder minutes.

Root cause: Minutes is omitted by omitempty when 0, so the API sees an override reminder without minutes.

Fixes #302

Notes

• includes a small compile-fix commit (contacts_crud helper signature types) so this branch remains CI-green against current main.

Testing

• go test ./...

[steipete]

Landed via temp rebase onto main.

• Gate: make fmt && go test ./internal/cmd -run 'TestBuildReminders|TestCalendarCreateCmd_ReminderPopupZeroForceSendsMinutes|TestCalendarUpdateCmd_SendUpdates' && make lint && make test && make worker-ci && make ci
• Land commit: 9cd6169
• Merge commit:

Thanks @salmonumbrella!

[steipete]

Merge finalized after GitHub recalculated mergeability post-force-push.

• Land commit: 9cd6169
• Merge commit: b8df361

Thanks again @salmonumbrella!