Replace slot pod edit icon with Spoolman link indicator dot

The pencil/eye icon was confusing (entire pod is already clickable).
Replaced with a 7px dot at the bottom of each pod:
- Green with soft glow = linked to Spoolman
- Dim white = not linked
Tooltip shows the linked spool ID on hover.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>