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/common.css

@@ -3,6 +3,7 @@
 @import "@hive/shared/base.css";
 @import "@hive/shared/terminal.css";
 @import "@hive/shared/tabs.css";
+@import "@hive/shared/chrome.css";
 
 /* ─── global typography ─────────────────────────────────────────────
    Element-level rules shared across all three pages (index, flow,
@@ -220,38 +221,9 @@ code {
   color: var(--fg);
 }
 
-/* ─── page chrome: header + back link (flow + logs pages) ───────────
-   .logs-header / .logs-back / .logs-title are used by both
-   /flow.html and /logs.html — the same frosted sticky bar pattern. */
-.logs-header {
-  position: sticky;
-  top: 0;
-  z-index: 25;
-  background: color-mix(in srgb, var(--bg) 92%, transparent);
-  -webkit-backdrop-filter: blur(8px) saturate(120%);
-  backdrop-filter: blur(8px) saturate(120%);
-  border-bottom: 1px solid var(--purple-dim);
-  display: flex;
-  align-items: center;
-  gap: 1.5em;
-  padding: 0.5em 1.5em;
-}
-
-.logs-back {
-  color: var(--purple);
-  text-decoration: none;
-  font-size: 0.88em;
-  white-space: nowrap;
-  flex: none;
-}
-.logs-back:hover { text-decoration: underline; }
-
-.logs-title {
-  color: var(--subtext0);
-  font-size: 0.85em;
-  letter-spacing: 0.05em;
-  flex: none;
-}
+/* page chrome (.page-header / .page-back / .page-title — the standalone
+   pages' sticky back-link bar) moved to @hive/shared/chrome.css, imported
+   at the top of this file. */
 
 /* ─── operator inbox + message rows ────────────────────────────────
    .inbox and .msg-* are rendered by common.js (renderInbox) and