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/stats.css

@@ -1,6 +1,6 @@
 /* ─── /stats.html — hive-wide turn-stats rollup ────────────────────
    Extracted from the dashboard ST4TS tab. Same minimal-
-   chrome pattern as /flow.html and /logs.html: the back-link header +
+   chrome pattern as /flow.html: the back-link header +
    title (.page-header / .page-back / .page-title) live in
    @hive/shared/chrome.css.
    `.hive-stats-table` also lives in common.css — it's shared with the