Backend: add/subtract/multiply/divide/scale operations, 2D FFT with power
spectrum, bandpass and Butterworth lowpass filters, RGB-to-HSV and RGB-to-Lab
conversions, split/merge channels, and Hyperstack::zip_map for two-image ops.
Frontend: ImageCalculator modal, ProcessMenu with all new operations, updated
Toolbar and store hooks. 70 tests passing.
Co-Authored-By: Claude Opus 4.6 <[email protected]>