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

docs/web-ui/css-vars.md

@@ -12,7 +12,7 @@ swap touches only the first:
    what the app references and it **never changes on a swap**.
 
 Both files live in `frontend/packages/shared/src/`. Per-page stylesheets
-(`common.css`, `dashboard.css`, `flow.css`, `logs.css`, `agent.css`)
+(`common.css`, `dashboard.css`, `flow.css`, `agent.css`)
 reference the **semantic** names and must **not** redeclare them or reach
 for the raw `--baseNN` slots directly. (`base.css` holds only the shared
 `body` typography — it references the palette but no longer defines it.)