fix: resolve calendar drag-and-drop not persisting post date changes

[GaryJones]

Summary

• Fix calendar drag-and-drop silently failing to persist post date changes
• Add defensive coding so AJAX requests proceed even if React store unavailable
• Add regression tests for same-timestamp post retrieval

Problem

When dragging posts on the Edit Flow calendar, the JavaScript crashed with:

Cannot read properties of null (reading 'setCalendarIsLoading')

The root cause was that calendar.js used dispatch('edit-flow/calendar') but the React bundle that registers this data store wasn't listed as a dependency. The store was undefined when the jQuery code ran, causing the crash before the AJAX request was made.

Solution

1. Script dependency fix (calendar.php): Reordered script enqueue so the React bundle loads before calendar.js, and added edit-flow-calendar-react-js as an explicit dependency.

2. Defensive coding (calendar.js): Added getCalendarDispatch() helper that returns no-op functions if the store isn't available, ensuring the AJAX request always proceeds.

3. Regression tests (CalendarModuleTest.php): Added tests verifying that posts with the same timestamp are correctly retrieved (the original reported symptom).

Test plan

• Drag a post to a different day on the calendar
• Verify the AJAX request is made (check Network tab)
• Click "Reset" and verify the post stays on the new date
• Drag multiple posts to the same day at the same time
• Verify all posts appear after page refresh
• Run composer test:integration -- --filter=CalendarModuleTest (24 tests pass)

Fixes #770

🤖 Generated with Claude Code