dashboard(matrix-accounts): populate agent picker from state containers

The agent dropdown read a non-existent `agents` field on /api/state, so
it always rendered empty ("— no agents —"). The endpoint exposes the
live roster under `containers` (each entry an object with `.name`).
Read that instead; the existing string-or-object map keeps it robust.

frontend/packages/dashboard/src/matrix-accounts.js

@@ -31,7 +31,10 @@ async function loadState() {
     if (!resp.ok) return;
     const s = await resp.json();
     renderServerWarnings(s.server_warnings);
-    agents = (s.agents || [])
+    // `/api/state` exposes the live roster under `containers` (each entry an
+    // object carrying `.name`); there is no top-level `agents` field, so the
+    // picker stays compatible with both string + object shapes defensively.
+    agents = (s.containers || [])
       .map((a) => (typeof a === 'string' ? a : a && a.name))
       .filter(Boolean)
       .sort();