Fix: Incorrect month display in Month view due to day-of-month comparison

[srihamat]

Description:

Problem

Tasks in Month view are incorrectly positioned 1 month earlier when the task's end date day-of-month is less than the Gantt chart's start date day-of-month.

Example:

• Task: 2026-02-18 → 2026-03-02
• Expected: Display in March
• Actual (buggy): Display in February

Root Cause

The diff() function in date_utils.js applies a day-of-month comparison (if (date_a.getDate() < date_b.getDate())) that incorrectly adjusts month calculations used for positioning in Month view.

Solution

• Separate month-scale calculations from day-scale calculations
• For month/year scales: Use pure year/month arithmetic
• For other scales: Keep fractional day-based adjustments

Testing

• ✅ Month view: Tasks display in correct month
• ✅ Day/Week views: No regression, still work correctly
• ✅ Backward compatible: Doesn't break existing functionality

Changes

• Modified diff() function in src/date_utils.js (lines 126-169)
• Added conditional logic to skip day-of-month adjustment for month-scale calculations