feat: dB scale modes (relative/absolute dBFS), about dialog, fix white-out

dB floor handling:
- Fix the white-out/inversion when the floor was cranked up: guard the
  (ceiling - floor) range in ColorizeSpectrogram so floor >= ceiling
  degrades to a hard threshold instead of dividing by ~0 (white) or a
  negative (inverted colors).
- Add two amplitude scale modes, toggled in the sidebar:
  * Relative: ceiling tracks the signal peak, floor sits N dB below it
    (slider = dynamic range, 10..100 dB). Floor can't cross the ceiling.
  * Absolute: fixed dBFS scale (0 dBFS = full scale), slider sets an
    absolute floor in dBFS. Brightness reflects real level.
  Mode + both slider values persist across loads. This replaces the
  amplitudeUserSet flag — storing the intent (range / absolute floor)
  preserves it across re-scales structurally.

About / Help dialog (no help menu existed):
- F1 or sidebar button; documents the scale modes and the caveats
  (dBFS not dBm — a WAV has no power reference; ~6 dB Hann window
  offset; pixels are reassigned energy, not raw bins) plus key list.
- Modal: underlying spectrogram input is gated while open; open/close
  handled in the main loop so the opening click can't self-close it and
  a dismissing click can't fall through into a selection.

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

src/render.c

@@ -200,10 +200,16 @@ void ColorizeSpectrogram(Image* image, Texture2D* texture)
     *image = GenImageColor(width, height, BLACK);
     Color* pixels = (Color*)image->data;
 
+    // Guard the range: if the floor reaches/exceeds the ceiling the division
+    // would otherwise go to ~0 (everything clamps white) or negative (the
+    // colors invert). A >=1 dB range degrades gracefully to a hard threshold.
+    float range = app.amplitudeCeilingDb - app.amplitudeFloorDb;
+    if (range < 1.0f) range = 1.0f;
+
     for (int i = 0; i < width * height; i++) {
         if (accumBuffer[i] > 0.0001f) {
             float db = AmplitudeToDecibels(accumBuffer[i]);
-            float normalized = (db - app.amplitudeFloorDb) / (app.amplitudeCeilingDb - app.amplitudeFloorDb);
+            float normalized = (db - app.amplitudeFloorDb) / range;
             normalized = Clamp(normalized, 0.0f, 1.0f);
             pixels[i] = GetColormapColor(normalized, app.colormap);
         }