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/builds.html

@@ -12,9 +12,7 @@
   </head>
   <body class="builds-shell">
     <!-- BU1LDS: the build lifecycle hub — rebuild queue, live build log,
-       meta inputs, and build log history on one page. Carved out of the
-       old /core.html (rebuild queue + meta inputs) and /logs.html (BUILD
-       tab) so the full build lifecycle is in one place. Same minimal
+       meta inputs, and build log history on one page. Same minimal
        chrome as /core.html — a `← home` back-link + a <hive-tab-strip>
        sub-tab nav. Default sub-tab is the rebuild queue. -->
     <header class="page-header">
@@ -74,7 +72,7 @@
         </div>
       </section>
 
-      <!-- BUILD L0GS: all-agents build log history (moved from /logs.html).
+      <!-- BUILD L0GS: all-agents build log history.
          Click a row to expand stdout + stderr. Live builds stream in real
          time. Lazy-loaded on first tab activation; auto-refreshes when
          rebuild_queue_changed fires. Deep-link: ?id=N#buildlogs. -->