feat(althing): surface the post office on Homepage under Toolchain

Labels the container into `Toolchain`, an existing group under the
existing Toolchain tab -- "the plumbing", which is where a message bus
belongs. Confirmed live: Homepage's API now returns it.

I had previously recorded in this file that no group fitted, which was
wrong. That conclusion came from a grep over the layout block that
missed the nested groups, and it went into a comment as though it were
a finding. The group was there the whole time.

Labels bind at container creation, so this deployed with `up -d` rather
than `restart`; a restart leaves the old labels and the dashboard keeps
showing what was there before. nh3-docker is already a discovered host
in homepage's docker.yaml as `nh3-pfi-docker`, so the label alone is
enough -- adding a services.yaml entry as well would render the card
twice.

althing-chamber on ana-docker also carries Toolchain labels and is a
separate service per the operator. Left alone.

stacks/althing-post-office/compose.yaml

@@ -128,11 +128,25 @@ services:
       retries: 3
       start_period: 10s
 
-    # No homepage labels, deliberately. There is an operator page at :8390/, but
-    # no group in stacks/homepage/conf/settings.yaml's `layout:` fits it, and a
-    # group the layout has never heard of gets no `tab:` and renders on ALL FOUR
-    # tabs (the Scriberr "AI Systems" bug, 2026-08-23). Adding a group is a
-    # separate, deliberate change — not a side effect of moving a container.
+    # `Toolchain` is an EXISTING group under the existing Toolchain tab in
+    # stacks/homepage/conf/settings.yaml — "the plumbing", which is where a
+    # message bus belongs. Naming a group the layout has never heard of gets no
+    # `tab:` and renders the group on ALL FOUR tabs (the Scriberr "AI Systems"
+    # bug, 2026-08-23), so this must stay a group that already exists.
+    #
+    # ⚠ Labels bind at container CREATION. Editing them needs `up -d`, never
+    # `restart` — a restart leaves the old labels in place and the dashboard
+    # keeps showing whatever was there before.
+    #
+    # nh3-docker is a discovered Docker host in homepage's docker.yaml (as
+    # `nh3-pfi-docker`), so the label is enough; do NOT also add a services.yaml
+    # entry or the card renders twice.
+    labels:
+      - homepage.group=Toolchain
+      - homepage.name=althing post office
+      - homepage.icon=mdi-mailbox
+      - homepage.description=althing v3 message bus — handles, unread counts, node liveness
+      - homepage.href=http://10.100.50.40:8390
 
 volumes:
   post-office-data: