Container link URLs (/screen.html, /stats.html) were root-relative,
which works when the agent is accessed directly at :port/ but resolves
to the gateway root when accessed via /agent/<name>/.
Fix at the source: harness emits relative URLs (screen.html, stats.html)
and the dashboard joins containerBase + '/' + url. The agent page uses
the URL directly and relative resolution is now correct in both paths.
c353dc1893