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/"

frontend/packages/shared/src/chrome.css

@@ -0,0 +1,45 @@
+/* ─── page chrome: back-link header ────────────────────────────────
+   The minimal sticky "frosted bar" header used by every standalone page
+   reached from the H0M3 hub — /flow, /logs, /stats, /settings. A back
+   link to H0M3 on the left, then either a page title or (on /logs) a
+   createTabStrip sub-tab nav. The operator dashboard uses its own richer
+   sticky chrome (`.dashboard-chrome`); these classes cover the simple
+   pages.
+
+   Shared across packages via @hive/shared so a new page only links the
+   stylesheet + writes the three-element header markup:
+
+     <header class="page-header">
+       <a class="page-back" href="/">← home</a>
+       <span class="page-title">TITLE</span>
+     </header>
+*/
+.page-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;
+}
+
+.page-back {
+  color: var(--purple);
+  text-decoration: none;
+  font-size: 0.88em;
+  white-space: nowrap;
+  flex: none;
+}
+.page-back:hover { text-decoration: underline; }
+
+.page-title {
+  color: var(--subtext0);
+  font-size: 0.85em;
+  letter-spacing: 0.05em;
+  flex: none;
+}