Fix websocket connection

[MaddieWright]

Fix websocket connection

What?

• the filter node was opening its own separate WebSocket connection solely to send the processing config, while the signal graph node opened a different connection to receive data
• the backend (handle_connection) always reads the first message as the processing config before starting the broadcast
• so the signal graph's connection would hang indefinitely waiting for a config that never arrived on it

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How?

• removed useWebsocket(0, 0) from filter-node.tsx. Instead, the filter node dispatches a processing-config-update custom window event with the current config whenever streaming starts or filter settings change
• added an event listener in useWebsocket that picks up processing-config-update events and forwards the config to the backend on the active connection
• changed onopen in useWebsocket to always send a default passthrough config if no filter config has been set yet, ensuring the backend always receives a first message and starts broadcasting
• fixed combo-box.tsx ignoring the lowCutoff/highCutoff/setLowCutoff/setHighCutoff props; the sliders were updating internal local state that was never passed back to the parent's buildConfig(), so filter cutoff values had no effect

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Testing

• confirmed Docker build succeeds for websocket-server after removing the Windows-only import
• verified in browser console that "WebSocket connection opened." logs on stream start
• confirmed no "Error parsing signal configuration JSON" errors appear in websocket-server logs
• tested Source to Chart View pipeline: data flows to chart on stream start
• tested Source to Filter to Chart View pipeline: data flows and filter type/cutoff changes dispatch config update events