dashboard: remove the L0GS page and its journal backend

Logs live at swarm level now (VictoriaLogs, read through Grafana
Explore). The hive dashboard's own journal viewer goes, with
everything that existed only to serve it:

- frontend: /logs.html, logs.js, logs.css, the H0M3 "Logs" tile and
  the per-agent menu's "journal logs ->" link. The build-log list
  rules BU1LDS used from logs.css move into builds.css.
- hive-c0re: GET /api/journal/{name} and GET /api/journal-host, and
  priv_client::read_container_journal.
- hive-priv / hive-priv-sock: the ReadContainerJournal request, its
  JournalQuery/JournalOutput wire types and the journalctl -M executor.
- nix: hive-c0re's systemd-journal supplementary group, whose only
  reader was the journal-host handler.
- docs: the L0GS section, the API entry and the security table row.

Closes #5113
Closes #3819

frontend/packages/dashboard/src/dashboard.html

@@ -90,11 +90,9 @@
         <!-- (Peer hives are no longer a tab — they render as a headline
            under SW4RM. See tab-pane-swarm.) -->
 
-        <!-- FL0W, L0GS, and the other standalone pages are reachable from
-           the H0M3 hub (served at /), not from the dashboard tab strip —
-           the strip holds only in-page tabs. Contextual deep-links into
-           the log viewer (an agent's logs, a build entry's log) live
-           inside the relevant dashboard content. -->
+        <!-- FL0W and the other standalone pages are reachable from the
+           H0M3 hub (served at /), not from the dashboard tab strip —
+           the strip holds only in-page tabs. -->
       </nav>
     </header>
 
@@ -335,8 +333,8 @@
       </p>
     </footer>
 
-    <!-- Slide-in detail panel (clicked file previews, journald logs,
-       applied config) is a <hive-side-panel> element
+    <!-- Slide-in detail panel (clicked file previews, applied config)
+       is a <hive-side-panel> element
        (@hive/shared/side-panel.js) — the Panel singleton in common.js
        creates + appends it to <body> lazily on first use, so nothing
        needs to be pre-declared here. -->