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/common.css

@@ -10,8 +10,8 @@
 @import "@hive/shared/pill.css";
 
 /* ─── global typography ─────────────────────────────────────────────
-   Element-level rules shared across all three pages (index, flow,
-   logs). These must not reference page-specific chrome classes. */
+   Element-level rules shared across every page. These must not
+   reference page-specific chrome classes. */
 
 h1,
 h2 {
@@ -152,8 +152,7 @@ code {
 }
 
 /* ─── buttons ───────────────────────────────────────────────────────
-   .btn base + semantic colour/size modifiers. Logs page uses
-   .btn-restart for the ↻ refresh buttons; dashboard uses all variants. */
+   .btn base + semantic colour/size modifiers. */
 .btn {
   font-family: inherit;
   font-weight: bold;
@@ -516,7 +515,7 @@ code {
 }
 
 /* ─── side panel ─────────────────────────────────────────────────
-   Long content (file previews, diffs, journald, applied config) opens in
+   Long content (file previews, diffs, applied config) opens in
    a drawer that swipes in from the right — used by both /flow.html and
    the dashboard. The drawer chrome itself (backdrop, drawer, resize
    handle, header, title, close button) is now the shared
@@ -605,8 +604,8 @@ hive-side-panel .md td {
    carries the horizontal gutter. Full-width chrome (the tab strip, the
    server-warnings banner, the footer) then spans edge-to-edge for free,
    with no negative-margin breakout hacks. Only matches on pages that opt
-   in by adding the wrapper (dashboard.html, index.html); FL0W / L0GS /
-   ST4TS are full-bleed with their own `.<page>-main` padding. */
+   in by adding the wrapper (dashboard.html, index.html); FL0W / ST4TS
+   are full-bleed with their own `.<page>-main` padding. */
 .page-content {
   padding: 0 1.5em;
 }