Introduce a new dropdown to control how fast the peak-hold line
decays on the live spectrum. Options:
Off — peaks never decay (manual reset only)
Slow — slow fade (0.005/frame)
Medium — moderate fade (0.015/frame, matches previous fixed rate)
Fast — quick fade (0.03/frame)
Previously the decay rate was hardcoded at 0.015/frame.
Add Hamming, Bartlett, Gaussian, and Kaiser window options to the
live spectrum display. Previously only None, Hanning, Blackman-Harris,
and Flat Top were available. The new windows map to different visual
power-curve scalings in the render loop.
The window function only affects the realtime spectrum visualization,
not the static psychoacoustic analysis (which always uses Hanning).
