Replace zoom FFT peak search with a budget-bounded PyFFTW direct transform

Drop the coarse+fine zoom refinement, the SciPy FFT backend, and the
exact= audit path in favor of a single always-on full-transform peak
search (_peak_bins). Block size is now derived from a per-thread memory
budget (_fft_block_for/SRAS_FFT_PLAN_BUDGET_MB) instead of a fixed
constant, so the existing block-parallel PyFFTW pool stays memory-safe
at high pad factors without the zoom algorithm's bookkeeping. Also
removes the now-unused threadpoolctl dependency and the FFT backend
selector from the UI.

Also includes a pre-existing min_freq_mhz peak-search floor (excludes
bins below a caller-supplied frequency from the argmax) that was
already implemented and tested in the working tree.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sras_viewer/common.py

@@ -26,6 +26,10 @@ CH1_DERIVED_MODES = (CH1_IDX, VELOCITY_MODE_IDX)
 
 CMAPS = ["gray", "viridis", "plasma", "inferno", "hot", "jet", "RdBu_r", "seismic"]
 
+# Fill color for masked (below-DC-threshold) pixels when "Highlight masked
+# pixels" is on, chosen to stand out against every colormap in CMAPS above.
+_MASKED_HIGHLIGHT_COLOR = "magenta"
+
 # (mode_str, status-bar unit, colorbar label) per channel index
 _CHANNEL_DISPLAY = {
     CH1_IDX: ("RF", "Peak frequency (MHz)", "MHz"),