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.js

@@ -292,12 +292,10 @@ export function openStream(url) {
   return target;
 }
 
-// Stream a build log into a <pre>, returning the EventSource. Shared by the
-// L0GS page BUILD tab and the C0R3 rebuild-queue live-log panel so the
-// append / sticky-scroll / stderr-separator / reconnect-replay logic lives in
-// one place. Appends `stdout_append` then `stderr_append` (one `--- stderr ---`
-// separator) frames from `GET /api/build-logs/id/{id}/stream`; auto-scrolls to
-// the bottom unless the operator scrolled up; on the terminal `done` frame
+// Stream a build log into a <pre>, returning the EventSource. Used by the
+// BU1LDS page's BUILD L0GS tab. Appends `stdout_append` then
+// `stderr_append` (one `--- stderr ---` separator) frames from
+// `GET /api/build-logs/id/{id}/stream`; auto-scrolls to the bottom unless the operator scrolled up; on the terminal `done` frame
 // closes the stream and calls `onDone(status)`; on a non-transient error
 // closes and calls `onError()`. The backend replays accumulated output on each
 // (re)connect, so a CONNECTING reconnect resets the <pre> to avoid doubling.
@@ -348,7 +346,7 @@ export function openBuildLogStream(id, pre, { onDone, onError } = {}) {
 
 // ─── side panel ─────────────────────────────────────────────────────────
 // Singleton drawer that swipes in from the right. Long content
-// (file previews, journald logs, applied config)
+// (file previews, applied config)
 // opens here via `sidePanel.open(title, node)` instead of expanding
 // inline. Body is swapped on each open; closing just slides out so
 // the content stays visible through the transition.
@@ -654,7 +652,7 @@ export const NOTIF = (() => {
 
 // ─── server warnings banner ──────────────────────────────────────────
 // A generic top-of-page banner shown on every page (dashboard + the
-// stand-alone FL0W / L0GS / H0M3 pages). The backend decides what to
+// stand-alone FL0W / H0M3 pages). The backend decides what to
 // warn about — `/api/state.server_warnings` is a list of
 // `{ kind, level, message }` — and this just renders it, coloured by
 // `level` (`warn` amber / `crit` red). Adding a new system warning is a