Deck comment mentions do not trigger notifications (users vs user)

Describe the bug

Mentions in Deck card comments are parsed correctly, but no notification is created for the mentioned user.

Deck 1.18.4 checks in NotificationHelper::sendMention():

if (($mention['type'] ?? 'users') !== 'users') {
    continue;
}

However, Nextcloud returns the mention type user, not users, so normal user mentions are skipped.

To Reproduce

1. Create a Deck board and share it with another user, e.g. testuser.
2. Create a card.
3. Add a comment containing @testuser mention test.
4. Log in as testuser.
5. No notification is shown.

The comment API correctly recognizes the mention:

"mentions": [
    {
        "mentionId": "testuser",
        "mentionType": "user",
        "mentionDisplayName": "Test User"
    }
]

Other notifications work correctly:

• sharing a Deck board
• assigning/sharing a Deck card
• mentioning the same user in a regular Nextcloud file comment

Expected behavior

The mentioned user should receive a Nextcloud notification.

Screenshots

Not applicable.

Client details:

• Device: desktop
• Browser: not relevant; reproducible server-side

if (($mention['type'] ?? 'users') !== 'users') {

if (($mention['type'] ?? null) !== 'user') {

Start at NotificationHelper::sendMention(), where the issue identifies the mention-type check, and compare its expected value with the comment API payload. Verify the change against the Deck comment reproduction using a user mention; done means the mentioned user receives a Nextcloud notification without affecting other notification types.