Refactor frontend websocket

[MaddieWright]

Websocket refactor

What?

refactored the websocket architecture to use a shared connection across all nodes, and fixed several related issues: filter cutoff values not being applied, chart time axis showing raw ISO timestamps, and chart data clearing when streaming is paused.

---

How?

Shared WebSocket context (WebSocketContext.tsx):

• previously useWebsocket created a separate connection per node
• the filter node had its own connection to send processing config, and the signal graph had its own connection to receive data; so config was never applied to the data stream
• now a single WebSocketProvider owns the one connection for the entire application
• it uses a pub/sub pattern: nodes subscribe to receive data points, and the filter node dispatches a processing-config-update custom event that the provider picks up and forwards to the backend

Additionally:

• useNodeData.ts replaces useWebsocket for nodes -> it subscribes to the shared data stream and windows/buffers data at the node's own rate (chartSize, batchesPerSecond), with no websocket logic of its own
• filter cutoff fix (combo-box.tsx): the lowCutoff/highCutoff/setLowCutoff/setHighCutoff props were declared in the interface but never destructured, so the sliders updated internal local state that was never passed back to the parent's buildConfig() -> fixed by making the component fully controlled
• buffer management (useNodeData.ts): buffer is cleared on stream start/stop to prevent backlog buildup and renderData is never cleared so the chart holds its last frame when paused and new data naturally replaces it on resume
• time formatting (WebSocketContext.tsx): backend sends ISO 8601 timestamps (2026-03-11T03:55:22Z) -> formatted to HH:MM:SS in normalizeBatch so both the chart axis and data table show readable times.

---

Testing

• verified Docker build succeeds for websocket-server
• confirmed only one websocket connection opens in browser console when streaming starts
• confirmed chart renders data with Source to Chart View pipeline
• confirmed filter config is sent on stream start with Source to Filter to Chart View pipeline
• confirmed changing filter type/cutoff while streaming dispatches updated config (visible in backend logs)
• confirmed chart holds last frame when stream is paused and resumes correctly
• confirmed time axis shows HH:MM:SS instead of full ISO timestamp