Surfaces the raw-amplitude classifier's per-node decision in
node_features[].classification so the UI can show which sensor is
actually seeing motion at any moment. Lets the operator visually find
the best sensor placement without physically moving things — just walk
around and watch which badge lights up.
Server side: adds amp_node_level() pure helper + amp_node_snapshot()
that reads AMP_LATEST, then plugs it into build_node_features so the
existing PerNodeFeatureInfo.classification carries the new labels.
UI: adds a global badge in the top bar and a per-node badge inline in
each h2, color-coded (grey/absent, blue/present_still, green/moving,
red/active) plus the live per-node CV %.
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