Add "Completion Date" (Mark as Done timestamp) to CSV export

Feature Description

Currently, when exporting a board to .csv, the Completion Date of a card is not included. After clicking the “Mark as done” button, the modification date of the card is updated, but there is no way to distinguish this event in the export.

Steps / Use Case

1. A user creates cards and sets due dates.
2. The card is completed by clicking “Mark as done”.
3. The board is exported to .csv.
4. The exported file does not contain any field showing the completion date (when the card was marked as done).

Expected Behavior

The CSV export should include a dedicated column (e.g., completion_date) with the timestamp of when the card was marked as done.

Actual Behavior

The only available date is the general modification date, which is not sufficient to track completion accurately. The only workaround is to manually copy this information into the card description.

System Information

Nextcloud Deck: 1.14.5
Nextcloud: 30.0.13.1
Browser/OS: Chrome (tested on Chrome 139.0.0.0). Linux 6.1.0-35-amd64 https://github.com/nextcloud/deck/pull/1 SMP PREEMPT_DYNAMIC Debian 6.1.137-1 (2025-05-07) x86_64

Proposed Solution

Add a completion_date column to the CSV export that records the exact time when the card was marked as done.

Additional Context

• This would help teams track real completion times for reporting and analysis.
• Currently, storing this information in card descriptions is inefficient and error-prone.

The issue names no files or tests. Start by locating the board CSV export and the “Mark as done” handling, then trace how card timestamps reach the export. Done means the CSV contains a dedicated completion_date column with the timestamp from marking a card done, while unfinished cards have an appropriate empty value.