[18.0][FIX] web_timeline: set date_stop to end of day for Date fields

[dmoranp]

When using Date fields (not DateTime) for date_stop in a timeline view, items end one day before the expected end date. The date_stop is parsed as 00:00:00 of that day, so the timeline item ends at the start of the day instead of including the full day.

The fix checks if the date_stop field type is date and sets it to end of day using Luxon's .endOf("day"), so the item correctly spans through 23:59:59 of the last day.

Steps to reproduce:

1. Create a model with Date fields for start and end dates
2. Create a timeline view using those fields
3. Create a record with date_start = 2026-03-10 and date_stop = 2026-03-14
4. The timeline item ends on 2026-03-13 instead of 2026-03-14

Fixes #3349