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/stats.css

@@ -1,7 +1,8 @@
 /* ─── /stats.html — hive-wide turn-stats rollup ────────────────────
    Extracted from the dashboard ST4TS 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.
+   title (.page-header / .page-back / .page-title) live in
+   @hive/shared/chrome.css.
    `.hive-stats-table` also lives in common.css — it's shared with the
    dashboard SYST3M › C0NT41N3R L04D table. Only the ST4TS-specific
    window selector, summary chips, and bars are page-specific. */