Mistyped User Tags in Shoutbox Link to 404

As discussed internally.

When a user mistypes a username while tagging someone in the shoutbox (e.g., tagging "AllenD" instead of "AllanD"), the tag generates a clickable link, but it redirects to a 404 error page.

Steps to Reproduce:
See shoutbox.
Attempt to tag a user using an incorrect username (e.g., @AllenD instead of @AllanD).
Observe that the tag creates a clickable link.
Click the link, and note that it redirects to a 404 error page.

Expected Behavior:
If the username does not exist, the tag should not create a link.
Alternatively, the system could validate the username and notify the user if it’s invalid.

Additional Notes:
It’s unclear if tagging in the shoutbox triggers a notification for the user being tagged.
Validation of tags before link creation could help address this issue.

It seems this is not limited to the shoutbox, but rather the whole software.