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/dashboard.css

@@ -32,9 +32,10 @@ body.dashboard-shell {
   margin: 0 0 1em;
 }
 /* ← home back-link to the H0M3 hub. Small purple link top-left of the
-   chrome, mirroring the .logs-back treatment the sub-pages use so the
-   navigation reads consistently. Full chrome unification (a shared,
-   reusable tab/back-link component) is a later step. */
+   chrome, mirroring the .page-back treatment the standalone pages use
+   (now shared from @hive/shared/chrome.css) so the navigation reads
+   consistently. The dashboard keeps this richer sticky chrome of its
+   own rather than folding into the simple page header. */
 .dash-home-back {
   display: inline-block;
   color: var(--purple);