Closes#4053
This started as a small fix.
After feedback and more debugging, I found more issues and
inconsistencies and went a bit down the rabbit hole.
The PR is bigger now, but I think the result is better: behavior is more
predictable, and the output is more consistent.
## Changes
- Multi-day full-day events now show an end date in `relative` and
`dateheaders` when `showEnd` is enabled.
- With `absolute` + `nextDaysRelative`, full-day events now keep the end
date when the start is replaced by TODAY/TOMORROW/etc.
- Timed events in `absolute` now also respect
`showEndsOnlyWithDuration`.
- Tests were expanded and refactored to cover more showEnd cases and
keep the setup easier to maintain.
- I also refactored parts of the calendar time rendering to reduce
duplicate logic.
## Before
<img width="1816" height="756" alt="before"
src="https://github.com/user-attachments/assets/ebec81fd-0c4a-4f9f-bbe3-e2b32ef6756e"
/>
## After
<img width="1816" height="756" alt="after"
src="https://github.com/user-attachments/assets/8a2c652d-dddc-4f6b-9074-fbef3411f9ed"
/>
