homepage: function-first layout + fleet label sweep

Reorganize the gethomepage dashboard from site-based (PFI-ANA, ESH, NH3)
to function-first grouping (Monitoring, AI Systems, Apps, Media, Games,
Infra-<site>, Service Networking). Canonical config now tracked in
configs/homepage/ with Plex/Jellyfin widget keys moved to env
substitution.

Label sweep across fleet compose files:
  - beszel, dozzle, backrest -> Monitoring
  - rest-server-ana -> Service Networking

Healthcheck fixes (previous wget/curl paths broke on distroless +
--private-repos 401):
  - beszel hub:    /beszel health --url ...
  - beszel agent:  /agent health (newly added)
  - rest-server:   nc -z localhost 8000 (TCP probe)

Group name originally "Wiring / Plumbing" collapsed to single-word group
on homepage's parser; renamed to "Service Networking" everywhere.

configs/homepage/custom.css

@@ -0,0 +1,2 @@
+/* Custom styling for homepage. Empty placeholder — keep file present so
+ * homepage doesn't 404 on the asset. */