calculadora

ci: add local AppImage packager and automated release workflow

perf: optimize release compilation profile to opt-level 3

feat(ui): add Signals/ODE help panel section; update TODO phases 3-4

feat(engine): add ode() RK4 solver — phase 4

feat(engine): add fft, fft_mag, ifft, conv, hann/hamming/blackman — phase 4 signal