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/stft.c

@@ -369,23 +369,28 @@ int ComputeSkipFactor(float signalDurationSec)
 // This replaces existing overview (skipFactor-strided) segments with
 
 // ===== Amplitude auto-scaling =====
+// Derive the colorizer's floor/ceiling from the current scale mode and controls.
+// Called after the STFT changes (load, background completion, FFT-size change)
+// and when the user toggles the mode. Deriving the floor from dynRangeDb /
+// absoluteFloorDb here is what preserves the user's setting across re-scales.
 void AutoScaleAmplitude(StftResult* stft)
 {
-    // Don't clobber a dB floor the user has set by hand. Reset on new file load
-    // re-enables auto-scaling (see ResetForNewSignal).
-    if (app.amplitudeUserSet) return;
+    if (app.amplitudeMode == SCALE_ABSOLUTE) {
+        // Fixed dBFS scale: 0 dBFS (full-scale) ceiling, user-set absolute floor.
+        app.amplitudeCeilingDb = 0.0f;
+        app.amplitudeFloorDb = app.absoluteFloorDb;
+        return;
+    }
 
+    // Relative: ceiling tracks the signal peak; floor sits dynRangeDb below it.
     float maxDb = -999.0f;
-    float minDb = 0.0f;
     for (int seg = 0; seg < stft->numSegments; seg++) {
         for (int bin = 0; bin < stft->segments[seg].numBins; bin++) {
             float db = AmplitudeToDecibels(stft->segments[seg].spectrum[bin].amplitude);
             if (db > maxDb) maxDb = db;
-            if (db < minDb) minDb = db;
         }
     }
-    // Set ceiling at the max, floor 40dB below — enough range to see structure
-    // but not so wide that the signal is drowned in black
+    if (maxDb < -998.0f) maxDb = 0.0f;  // no data yet — sane default
     app.amplitudeCeilingDb = maxDb;
-    app.amplitudeFloorDb = maxDb - 40.0f;
+    app.amplitudeFloorDb = maxDb - app.dynRangeDb;
 }