Builds on the merged dashboard theming: the themed-frontend overlay now
swaps colors.css in both the dashboard/ and agent/ dist subtrees, and the
agent frontend dir (HIVE_AGENT_FRONTEND_DIR, which the gateway serves agent
static files from) points at the themed tree too. So when stylix is enabled
on the host, both the dashboard and the gateway-served agent UIs re-theme
from the host palette — still zero-op auto-detect, still a no-op (both dirs
on the unthemed dist) when stylix isn't imported.
Not covered: an agent reached directly on its own harness web server (no
gateway) serves from its per-agent mergedDist, built in the agent's own
nixosSystem with no host stylix access — that path needs the base16 palette
forwarded host->agent and is tracked separately.
8006dd2809