agent ui: shared page-header nav on the stats + screen sub-pages

Per mara: the agent sub-pages should use the same back-link nav as the
dashboard's standalone pages, while the live terminal page stays as-is (no
tabs of each other, not an SPA).

Pull the shared `@hive/shared/chrome.css` into the agent bundle (via agent.css's
existing @import line; esbuild inlines it into the one dist/static/agent.css all
agent pages link), then:

- stats.html: replace the ASCII banner + bespoke `.stats-nav` with the shared
  `page-header` (← live back-link + dashboard link + title). Keeps the #back-link
  / #dashboard-link / #title ids stats.js drives, and the time-window picker +
  charts below are unchanged.
- screen.html: replace the bespoke `#toolbar` nav with the same `page-header`
  (← agent back-link + title), keeping the fit / match / debug controls + the
  status text in the bar.

The live terminal (index.html) is untouched. The now-unused `.banner` /
`.stats-nav` / `#toolbar` rules in agent.css are left for a follow-up prune to
keep this diff focused on the markup. Agent build clean.

Closes #1874.

frontend/packages/agent/src/agent.css

@@ -2,6 +2,10 @@
    Bundled in front of the agent-only rules below via esbuild. */
 @import "@hive/shared/base.css";
 @import "@hive/shared/terminal.css";
+/* Shared page-header chrome (page-header / page-back / page-title) — gives the
+   agent SUB-pages (stats, screen) the same back-link nav the dashboard's
+   standalone pages use. The live terminal page keeps its own header. */
+@import "@hive/shared/chrome.css";
 
 /* ─── full-screen layout overrides ─────────────────────────────────
    The agent page mounts a full-viewport terminal under a fixed