fix(web-app): truncate long plugin names in the admin sidebar nav

[newmanw]

Summary

• Long plugin titles in the admin sidebar nav wrapped onto a second line, but .nav-item has a fixed 36px height and no overflow handling, so the wrapped line spilled out and visually overlapped the nav item below it.
• Truncate the plugin label with an ellipsis instead of wrapping, scoped to a new .nav-label--truncate modifier class rather than the shared .nav-label class — .nav-label is also used by the Users/Devices nav items which carry a matBadge, and overflow: hidden there would risk clipping the badge indicator.

Test plan

• Edit a plugin nav label's text in devtools to something long and confirm it truncates cleanly with no overlap into the row below
• Confirm Users/Devices unread-count badges still render correctly