Add peak hold decay rate selector

Introduce a new dropdown to control how fast the peak-hold line
decays on the live spectrum. Options:
  Off   — peaks never decay (manual reset only)
  Slow  — slow fade (0.005/frame)
  Medium — moderate fade (0.015/frame, matches previous fixed rate)
  Fast   — quick fade (0.03/frame)

Previously the decay rate was hardcoded at 0.015/frame.

index.html

@@ -32,6 +32,12 @@
         <button id="resetBtn" class="player-btn" title="Reset Peaks">↺</button>
         <span id="playTime" class="player-time">0:00.0 / 0:00.0</span>
         <select id="windowSelect" class="window-select" title="Spectrum window function"><option value="none">None</option><option value="hanning" selected>Hanning</option><option value="hamming">Hamming</option><option value="blackman">Blackman-Harris</option><option value="bartlett">Bartlett</option><option value="gaussian">Gaussian</option><option value="kaiser">Kaiser</option><option value="flatTop">Flat Top</option></select>
+        <select id="peakHoldDecay" class="window-select" title="Peak hold decay rate">
+          <option value="off">Hold: Off</option>
+          <option value="slow">Hold: Slow</option>
+          <option value="medium" selected>Hold: Medium</option>
+          <option value="fast">Hold: Fast</option>
+        </select>
       </div>
       <div class="chart-container" style="position:relative">
         <canvas id="waveformChart"></canvas>

js/app.js

@@ -616,7 +616,9 @@ const App = {
     ctx.stroke();
 
     if (this.holdPeaks) {
-      const decay = 0.015;
+      const decaySel = document.getElementById('peakHoldDecay');
+      const decayMap = { off: 0, slow: 0.005, medium: 0.015, fast: 0.03 };
+      const decay = decaySel ? (decayMap[decaySel.value] ?? 0.015) : 0.015;
       ctx.beginPath();
       let started = false;
       for (let x = 0; x < chartW; x++) {