refactor(frontend): extract the page-chrome into @hive/shared (#1464 step 3)

The standalone pages (/flow, /logs, /stats, /settings) all share the same
sticky back-link header, but its CSS lived in the dashboard's common.css
under the misnomer `.logs-*` (the comment even noted it was used by more
than logs). Promote it to a first-class shared component.

- new @hive/shared/chrome.css with neutral `.page-header` / `.page-back` /
  `.page-title` (cohering with the `.page-content` gutter wrapper);
  exported from the shared package and @imported into common.css next to
  base/terminal/tabs.css.
- rename the four pages' header markup `.logs-*` → `.page-*`.
- drop the old `.logs-*` chrome block from common.css; refresh the stale
  doc comments in settings.css / stats.css / dashboard.css.

CSS-only, behaviour/visual-neutral (same rules, new names + home). The
dashboard keeps its own richer sticky `.dashboard-chrome`; unifying the
per-agent UI's `.stats-nav` chrome is a larger follow-up, deferred.

Part of #1464 step 3.

frontend/packages/dashboard/src/settings.css

@@ -1,9 +1,9 @@
 /* ─── /settings.html — operator-local preferences ──────────────────
    Extracted from the dashboard S3TT1NGS tab. Same
    minimal-chrome pattern as /flow.html and /logs.html: the back-link
-   header + title (.logs-header / .logs-back / .logs-title) live in
-   common.css; only the page body padding + the notif-toggle row are
-   page-specific and live here. */
+   header + title (.page-header / .page-back / .page-title) live in
+   @hive/shared/chrome.css; only the page body padding + the notif-toggle
+   row are page-specific and live here. */
 
 body.settings-shell {
   margin: 0;