add draw mask node

frontend/src/styles.css

@@ -568,6 +568,47 @@ html, body, #root {
   opacity: 0.9;
 }
 
+.mask-paint-overlay {
+  position: relative;
+  overflow: hidden;
+  user-select: none;
+  touch-action: none;
+  background: #0f172a;
+  border: 1px solid #334155;
+  border-radius: 6px;
+  cursor: crosshair;
+}
+
+.mask-paint-overlay-drawing {
+  cursor: crosshair;
+}
+
+.mask-paint-image {
+  width: 100%;
+  display: block;
+}
+
+.mask-paint-canvas {
+  position: absolute;
+  inset: 0;
+  width: 100%;
+  height: 100%;
+  pointer-events: none;
+}
+
+.mask-paint-cursor {
+  position: absolute;
+  border: 1.5px solid rgba(255, 255, 255, 0.95);
+  border-radius: 50%;
+  background: rgba(255, 255, 255, 0.08);
+  box-shadow:
+    0 0 0 1px rgba(239, 68, 68, 0.85),
+    0 0 10px rgba(15, 23, 42, 0.35);
+  transform: translate(-50%, -50%);
+  pointer-events: none;
+  z-index: 2;
+}
+
 /* ── 3D surface view ──────────────────────────────────────────────── */
 .surface-view-container {
   width: 100%;