Updates to CartContents

Previously a single undefined key crashed the .localeCompare sort, which — because it runs inside the CartContents render — blanked the entire cart list (and surfaced as TypeError: Cannot read properties of undefined (reading 'localeCompare')).

The grouping now coerces absent keys defensively (consistent with the package's untrusted-payload trust seam): every item still renders, dated events keep their correct order/grouping, and any undated events trail in a final group rather than taking down the whole drawer. Date headers/times for undated events fall back to empty via the existing render-layer guards.

Note: this is defence-in-depth. Correct ordering/day-grouping still requires the consuming app to supply eventStartAtUtc + eventDateKey on each event.