End date is always +1 day after export to .ics file

Steps to reproduce

1. Export a calendar

Expected behavior

Start and end dates of the exported .ics file match the entry in nextcloud calendar.

Actual behavior

The end date always seems to be 1 day off.

Example entry:

Exported ics file:

PRODID:-//IDN nextcloud.com//Calendar app 5.5.11+murena-20260107//EN
CALSCALE:GREGORIAN
VERSION:2.0
BEGIN:VEVENT
CREATED:20260329T112146Z
DTSTAMP:20260329T112407Z
LAST-MODIFIED:20260329T112407Z
SEQUENCE:2
UID:aaa423c2-3c0a-493b-a512-4a8690bbcf83
DTSTART;VALUE=DATE:19960706
DTEND;VALUE=DATE:19960721
STATUS:CONFIRMED
SUMMARY:Ferien Sitten
LOCATION:Sion
END:VEVENT
END:VCALENDAR``


**Note how the end dates don't match.**

### Calendar app version

Unknown (nextcloud 31, Murena Workspace build)

### CalDAV-clients used

none

### Browser

Zen 1.19.5b (Firefox 149)

### Client operating system

CachyOS

### Server operating system

Unknown (nextcloud 31, Murena Workspace build)

### Web server

None

### Database engine version

None

### PHP engine version

None

### Nextcloud version

31

### Updated from an older installed version or fresh install

None

### List of activated apps

```Text

Nextcloud configuration

Web server error log

Log file

Browser log

Additional info

No response

Start by tracing the calendar export path that produces the shown VCALENDAR and compare how all-day DTSTART and DTEND values are derived. Reproduce the example with an all-day event, then verify that the exported .ics end date matches the calendar entry and add or update coverage for this case.