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/shared/package.json

@@ -13,7 +13,8 @@
     "./colors.css": "./src/colors.css",
     "./theme.css": "./src/theme.css",
     "./base.css": "./src/base.css",
-    "./terminal.css": "./src/terminal.css"
+    "./terminal.css": "./src/terminal.css",
+    "./chrome.css": "./src/chrome.css"
   },
   "files": [
     "src/"