* Fix recurring task initial due date calculation to match recurrence pattern
Resolves#963
When creating a recurring task without an explicit due date, the system
was incorrectly setting it to "today" regardless of the recurrence pattern.
This caused issues where:
- Monthly tasks set to recur on a specific day (e.g., 28th) would show the
wrong due date (today/yesterday instead of the 28th)
- Tasks wouldn't appear correctly in the Upcoming view
- The base due_date didn't match the recurrence_month_day setting
Changes:
- Add calculateInitialDueDate() helper to compute correct first occurrence
- For monthly recurrence with specific day: calculate next occurrence of that day
- For weekly recurrence with specific weekday: calculate next occurrence of that weekday
- For other types (daily, etc.): maintain current behavior (use today)
- Apply same logic to both task creation and updates
Tests:
- Add comprehensive test suite (9 new tests) covering:
- Monthly recurrence with future day in current month
- Monthly recurrence with past day (should use next month)
- Weekly recurrence with specific weekday
- Daily recurrence (should still default to today)
- Edge cases (31st of month, explicitly provided dates)
- Task updates adding recurrence
All 54 recurring task tests pass.
* Fix UTC timezone bug in recurring task expansion and add comprehensive tests
- Fix expandRecurringTasks() to use setUTCHours instead of setHours
- Add 42 unit tests for recurringTaskService UTC consistency
- Add 24 DST transition tests (spring forward/fall back)
- Verify no occurrence skips or duplicates during DST
- Test month-end handling, leap years, and timezone boundaries
077addadde