improve: bandpass skirt width in Hz, not bins (consistent across selections)

The playback bandpass transition was freqPerBin*10 wide, and freqPerBin
depends on the FFT size, which is derived from the selection length — so
the same frequency band got a sharper or softer filter depending on how
long a region you selected. Set the skirt to ~20% of the passband (capped
at 100 Hz), floored at 3 FFT bins so it stays smooth at coarse resolution.
Now a given band sounds the same regardless of selection duration.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/audio.c

@@ -136,14 +136,23 @@ static void ApplyBandpassFilterFFT(float* samples, int numSamples, int sampleRat
     FFT(fftInput, fftOutput, fftSize, false);
     
     float freqPerBin = (float)sampleRate / fftSize;
+
+    // Transition (skirt) width in Hz: ~20% of the passband, capped at 100 Hz,
+    // with a floor of 3 FFT bins so it stays smooth at coarse resolution.
+    // Previously the skirt was tied to bin width alone (freqPerBin * 10), so the
+    // FFT size — and thus the filter's sharpness — changed with the selection
+    // length: the same band sounded different depending on how much you boxed.
+    float bandwidthHz = freqHigh - freqLow;
+    float transitionHz = fmaxf(fminf(bandwidthHz * 0.2f, 100.0f), freqPerBin * 3.0f);
+
     for (int bin = 0; bin < fftSize / 2 + 1; bin++) {
         float frequency = bin * freqPerBin;
         float attenuation = 1.0f;
         if (frequency < freqLow) {
-            float dist = (freqLow - frequency) / (freqPerBin * 10.0f);
+            float dist = (freqLow - frequency) / transitionHz;
             attenuation = 1.0f / (1.0f + dist * dist * dist);
         } else if (frequency > freqHigh) {
-            float dist = (frequency - freqHigh) / (freqPerBin * 10.0f);
+            float dist = (frequency - freqHigh) / transitionHz;
             attenuation = 1.0f / (1.0f + dist * dist * dist);
         }
         fftOutput[bin] *= attenuation;