﻿Add UI-configurable settings and backend endpoints.

Requirements:
- Add REST endpoints:
  - GET /api/config (already) and POST /api/config to update runtime config (center_hz, sample_rate, fft_size, gain_db, detector thresholds).
  - POST /api/sdr/settings to toggle AGC and DC/IQ correction flags.
- Add runtime-config application: when config updates are posted, apply to DSP loop and SDRplay source without full restart when possible.
  - For SDRplay: update center frequency and gain live (if API allows), else restart source gracefully.
  - For mock: just update values.
- Add DSP options:
  - DC block (simple IIR high-pass) toggle
  - IQ imbalance correction (basic amplitude/phase correction; can be minimal)
- Add UI controls panel (sliders/inputs) for center freq, span (sample rate), FFT size, gain, AGC, DC block, IQ balance, detector threshold.
  - Also add preset buttons for 40m, 20m, 17m.
  - Apply changes via API and reflect current config.
- Ensure the existing live spectrum/waterfall/timeline keep working.
- Update README with new UI config.
- Add unit tests for config update logic if possible.
- Run go test ./... and go build ./cmd/sdrd.

Note: stay framework-free, plain JS + Canvas.

When completely finished, run: openclaw system event --text "Done: Added UI-configurable SDR settings + DSP toggles" --mode now
