fix: point domain at /dashboard — engram cloud serves no route at /

engram `cloud serve` only registers /health, /sync/* and /dashboard/*;
Go's ServeMux answers "404 page not found" at the root path, which is
indistinguishable from a Traefik router miss. The deploy itself was fine
(both containers healthy), but the default domain pointed at `/` and
kept returning 404. Adding `path = "/dashboard"` routes the app's real
browser surface (login page / dashboard).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

blueprints/engram/template.toml

@@ -17,3 +17,4 @@ mounts = []
 serviceName = "engram"
 port = 8080
 host = "${main_domain}"
+path = "/dashboard"