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/shared/src/chrome.css

@@ -1,8 +1,8 @@
 /* ─── page chrome: back-link header ────────────────────────────────
    The minimal sticky "frosted bar" header used by every standalone page
-   reached from the H0M3 hub — /flow, /logs, /stats, /settings. A back
-   link to H0M3 on the left, then either a page title or (on /logs) a
-   createTabStrip sub-tab nav. The operator dashboard uses its own richer
+   reached from the H0M3 hub — /flow, /stats, /core, /builds. A back
+   link to H0M3 on the left, then either a page title or (on /core and
+   /builds) a sub-tab nav. The operator dashboard uses its own richer
    sticky chrome (`.dashboard-chrome`); these classes cover the simple
    pages.
 
@@ -24,10 +24,9 @@
   border-bottom: 1px solid var(--purple-dim);
   display: flex;
   /* Keep the back-link + sub-tab nav on one row. Without an explicit
-     nowrap a flex:1 sub-tab strip (e.g. /logs.html's `.logs-tabbar`)
-     whose intrinsic min-content width can't shrink (default
-     `min-width: auto`) gets pushed onto its own line. The strip pairs
-     this with `min-width: 0` so it can shrink instead of wrapping. */
+     nowrap a flex:1 sub-tab strip (e.g. `.core-tabbar`) whose intrinsic
+     min-content width can't shrink (default `min-width: auto`) gets
+     pushed onto its own line. */
   flex-wrap: nowrap;
   align-items: center;
   gap: 1.5em;