Gate spectral bypass on watermark detection

Blindly subtracting the codebook's carrier pattern from an image that
never had the watermark would imprint one instead of removing it. Now
each image is run through RobustSynthIDExtractor first, and the V3
bypass only runs when a watermark is actually detected; otherwise the
image passes through untouched (still eligible for metadata stripping).

gui/requirements-gui.txt

@@ -3,3 +3,5 @@ scipy
 opencv-python-headless
 pillow
 tkinterdnd2
+PyWavelets
+scikit-learn