fix(agent): use gateway-relative dashboard URL when behind hive-gateway

When the agent page is accessed via hive-gateway (path is /agent/<name>/)
the dashboard lives at the same origin's root (/), not at the direct
TCP port. Both app.js (overflow menu / setHeader) and stats.js
(dashboard-link anchor) previously hardcoded http://hostname:port which
is wrong or unreachable behind HTTPS TLS termination.

Detection: if location.pathname starts with '/agent/', use
location.origin + '/' as the dashboard base. Otherwise fall back to the
original direct-TCP construction for non-gateway deploys.

frontend/packages/agent/src/stats.js

@@ -330,7 +330,13 @@ window.Chart = Chart;
       document.title = 'stats · ' + s.label;
       document.getElementById('title').textContent = '◆ ' + s.label + ' ◆';
       const dl = document.getElementById('dashboard-link');
-      dl.href = 'http://' + window.location.hostname + ':' + s.dashboard_port + '/';
+      // When accessed via hive-gateway the page lives at `/agent/<name>/`
+      // on the same origin as the dashboard (`/`). Detect via path prefix
+      // rather than the direct port (which is unreachable or wrong scheme
+      // behind HTTPS TLS termination).
+      dl.href = window.location.pathname.startsWith('/agent/')
+        ? window.location.origin + '/'
+        : 'http://' + window.location.hostname + ':' + s.dashboard_port + '/';
     } catch (_) { /* non-fatal */ }
   }