fix: improve bpm reliability

perf: 15s calibration window and weighted-median final bpm

perf: optimize signal processing pipeline and cleanup frontend

perf: optimize shader background rendering

fix: update final heart rate calculation to use only recent readings

feat: migrate rPPG pipeline to FaceLandmarker and 5-method adaptive extraction

fix: enforce minimum autocorrelation search range of 50 lags regardless of fps

fix: simplify status machine to calibrating timeframe, not signal quality gate

fix: reset bpm history on measuring entry and raise min bpm to 50

fix: compute effective FPS from timespan to handle bursty WebSocket delivery

fix: improve BPM accuracy with first-peak autocorrelation and bandpass fix

feat: websocket endpoint, frontend utilities, face detection and pulse hook