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.
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