Fix incorrect month/year diff calculation in date_utils

[RickCarletti]

The diff function in date_utils was calculating differences incorrectly when the scale was set to month or year.
This caused items to be displayed in the wrong month when switching the view_mode to month or year.

Example of the issue

• Input: 2025-10-01
• Displayed as: 2025-09-01

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Changes

• Replaced the previous month/year calculation (which used an approximation of 30 days per month).
• Implemented a new diffInMonths function that:
  • Ensures correct handling of which date is greater.
  • Calculates exact whole months between dates.
  • Anchors the calculation on the real calendar month length.
  • Computes the fractional month based on actual month duration.

✅ With this fix, month and year differences are now calculated accurately and items display in the correct position.