[HADA] Issue comments, @mentions, and in-app notifications

Issue comments, @mentions, and in-app notifications are now available.

Hello from the Hada operations team.
You can leave progress notes on an issue before it’s resolved, @mention related admins, and open the exact comment from the dashboard bell.

✅ Leave comments and replies on issue details

A comment section is added at the bottom of the dashboard Issue detail.

• Owners, Super admins, and Managers can write and view comments.
• Edit is author-only. Delete is allowed for the author, Owner, and Super admin (Managers can delete only their own).
• Replies are one level deep. Opening a reply auto-inserts @name for the target author.
• Comments are limited to 500 characters; the input grows from 1 to 4 lines.

✅ Mention admins with @

Type @ while writing a comment or reply to open the admin list.

• Candidates are Owner, Super admin, and Manager.
• Use arrow keys and Enter to select.
• Mentioned names are highlighted in the comment body.

✅ Mentions appear in the dashboard bell

Check in-app notifications from the bell at the top right.

• Only mentioned admins receive a notification. (Self-mentions and comments without mentions do not notify.)
• Clicking a notification opens that issue and scrolls to the comment.
• This release covers dashboard in-app alerts only — email, Alimtalk, and Settings > Notifications are unchanged.