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.
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