- Fix Jalali/Gregorian/Hijri day-of-week startOffset formulas
(dow returns 0=Sat, not 0=Sun as the old comment claimed)
- Fix calendar and export month picker navigation jumping 2 months
(navMonth buttons already have pre-computed values; handlers
were decrementing/incrementing them again)
- Fix GetOrCreateUser/GetOrCreateDay race condition with INSERT OR IGNORE
- Add month/day bounds validation in handleEditMsg to prevent panic