fix(calendar): make showEnd behavior more consistent across time formats (#4059)

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"
/>

tests/mocks/event_with_time_over_multiple_days_yearly.ics

@@ -0,0 +1,18 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//MagicMirror Test//timed-multiday-yearly//EN
+CALSCALE:GREGORIAN
+BEGIN:VEVENT
+DTSTART:20241026T010000Z
+DTEND:20241026T110000Z
+DTSTAMP:20241024T153358Z
+UID:4maud6s79m41a99pj2g7j5km0a@google.com
+CREATED:20241024T153313Z
+LAST-MODIFIED:20241024T153330Z
+SEQUENCE:0
+STATUS:CONFIRMED
+RRULE:FREQ=YEARLY
+SUMMARY:Sleep over at Bobs
+TRANSP:OPAQUE
+END:VEVENT
+END:VCALENDAR

tests/mocks/event_with_time_same_day_yearly.ics

@@ -0,0 +1,18 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//MagicMirror Test//timed-same-day-yearly//EN
+CALSCALE:GREGORIAN
+BEGIN:VEVENT
+DTSTART:20241025T200000
+DTEND:20241025T220000
+DTSTAMP:20241024T153358Z
+UID:timed-same-day-yearly@magicmirror
+CREATED:20241024T153313Z
+LAST-MODIFIED:20241024T153330Z
+SEQUENCE:0
+STATUS:CONFIRMED
+RRULE:FREQ=YEARLY
+SUMMARY:Same day timed event
+TRANSP:OPAQUE
+END:VEVENT
+END:VCALENDAR