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/flow.html

@@ -16,9 +16,9 @@
        No full tabbar — the flow page is a dedicated full-viewport
        terminal surface; navigating back to the menu is the only chrome
        needed. Pages link back to H0M3, not the dashboard. -->
-  <header class="logs-header">
-    <a class="logs-back" href="/">← home</a>
-    <span class="logs-title">FL0W</span>
+  <header class="page-header">
+    <a class="page-back" href="/">← home</a>
+    <span class="page-title">FL0W</span>
     <select id="flow-agent-filter" class="flow-filter" title="filter timeline by agent" aria-label="filter timeline by agent">
       <option value="">all agents</option>
     </select>