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/core.html

@@ -14,7 +14,7 @@
     <!-- C0R3: the host/coordinator surface. Standalone page (served at
        /core.html) carved out of the dashboard's old SYST3M tab so the
        dashboard tab strip stays lean. Same minimal chrome as
-       /logs.html — a `← home` back-link to the H0M3 hub + a
+       /builds.html — a `← home` back-link to the H0M3 hub + a
        <hive-tab-strip> sub-tab nav. Two sub-tabs: kept state (tombstones)
        and container load. Rebuild queue + meta inputs have moved to
        /builds.html (the build lifecycle hub). Default tab: K3PT ST4T3.