feat(web): master gain slider with -/+ steppers

The web surface had no master control at all (SC-only via
/control/doMaster 0..2). Adds the slider next to Tempo with 0.05
steppers and a live readout, same pattern as the tempo control.

web_realart/public/control/index.html

@@ -38,6 +38,7 @@
   </div>
   <button id="clear" class="wide">Tout couper</button>
   <label>Tempo <button id="tempo-minus" class="tempo-step" type="button">-</button><input id="tempo" type="range" min="60" max="180" value="126"><button id="tempo-plus" class="tempo-step" type="button">+</button><span id="tempo-val">126</span></label>
+  <label>Master <button id="master-minus" class="tempo-step" type="button">-</button><input id="master" type="range" min="0" max="2" step="0.05" value="1"><button id="master-plus" class="tempo-step" type="button">+</button><span id="master-val">1.00</span></label>
   <div class="quant-bar">
     <span class="quant-label">Quant</span>
     <button class="quant-btn" data-quant="0.5">½</button>

web_realart/public/control/js/transport.js

@@ -42,6 +42,24 @@ export function init() {
   tMinus && tMinus.addEventListener("click", () => stepTempo(-1));
   tPlus && tPlus.addEventListener("click", () => stepTempo(1));
 
+  // Master gain: slider + -/+ steppers (0.05 per click), SC /control/doMaster (0..2)
+  const master = document.getElementById("master");
+  const masterVal = document.getElementById("master-val");
+  const pushMaster = () => {
+    if (masterVal) masterVal.textContent = (+master.value).toFixed(2);
+    send("/control/doMaster", +master.value);
+  };
+  master && master.addEventListener("input", pushMaster);
+  const stepMaster = (d) => {
+    if (!master) return;
+    master.value = Math.max(+master.min, Math.min(+master.max, +master.value + d)).toFixed(2);
+    pushMaster();
+  };
+  const mMinus = document.getElementById("master-minus");
+  const mPlus = document.getElementById("master-plus");
+  mMinus && mMinus.addEventListener("click", () => stepMaster(-0.05));
+  mPlus && mPlus.addEventListener("click", () => stepMaster(0.05));
+
   // Filter
   const filt = document.getElementById("filter");
   filt && filt.addEventListener("input", () => send("/control/fx/filter", +filt.value));