feat(cms): make-transparent (remove white background) in the logo editor

Adds a ⚪ Háttér átlátszóvá (fehér) button to the canvas editor. Pixels above
a luminance threshold become fully transparent; a soft ramp just below keeps
edges smooth. Lets white-background partner/site logos sit on any page
background.

Closes MITHOME-84

scripts/cms-logo-client.js

@@ -146,6 +146,36 @@ function toggleInvert() {
   render();
 }
 
+// WHY: many partner/site logos arrive with a solid (usually white) background.
+// Removing it lets the logo sit cleanly on any page background. Pixels brighter
+// than the threshold become transparent; a soft ramp just below it keeps the
+// edges smooth instead of jagged.
+function makeTransparent() {
+  if (!editImg.width) return;
+  const off = document.createElement('canvas');
+  off.width = editImg.width;
+  off.height = editImg.height;
+  const octx = off.getContext('2d');
+  octx.drawImage(editImg, 0, 0);
+  const imgData = octx.getImageData(0, 0, off.width, off.height);
+  const d = imgData.data;
+  const thresh = 235;   // fully transparent above this luminance
+  const soft = 30;      // smooth ramp below the threshold
+  for (let i = 0; i < d.length; i += 4) {
+    const lum = (d[i] + d[i + 1] + d[i + 2]) / 3;
+    if (lum > thresh) {
+      d[i + 3] = 0;
+    } else if (lum > thresh - soft) {
+      const t = (lum - (thresh - soft)) / soft; // 0..1
+      d[i + 3] = Math.round(d[i + 3] * (1 - t));
+    }
+  }
+  octx.putImageData(imgData, 0, 0);
+  const next = new Image();
+  next.onload = () => { editImg = next; render(); };
+  next.src = off.toDataURL('image/png');
+}
+
 function resetFilters() {
   editState.brightness = 100; editState.contrast = 100; editState.invert = false; editState.padding = 0;
   document.getElementById('bright-range').value = 100; document.getElementById('bright-val').textContent = '100%';