The dashboard vhost split static-vs-backend on the request Accept
header (map $http_accept $dashboard_spa_target), so the same URL
behaved differently by content-type — e.g. /api/state fetched with
Accept: text/html wrongly returned index.html.
Now that all hive-c0re routes live under /api/ plus the single
/webhook/knowledge endpoint, route by path instead: /api/ and
/webhook/ proxy to c0re (SSE settings on /api/), everything else
serves the dist with try_files $uri /index.html. Drops the
dashboard Accept-header map and the @c0re named location.
Updates docs/gateway.md accordingly.
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