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/logs.html

@@ -15,8 +15,8 @@
   <!-- Minimal chrome: back link + sub-tab strip.
        Same pattern as flow.html — no full dashboard tabbar. Back link
        points to the H0M3 hub (served at /), not the dashboard. -->
-  <header class="logs-header">
-    <a class="logs-back" href="/">← home</a>
+  <header class="page-header">
+    <a class="page-back" href="/">← home</a>
     <nav class="hive-tabbar logs-tabbar" id="logs-tabbar" role="tablist">
       <a class="hive-tab" id="logs-tab-build" href="#build" role="tab"
          aria-controls="logs-pane-build" data-tab="build">