Refactor GUI rendering and align RTA axis

www/core/canvas_surface.js

@@ -0,0 +1,18 @@
+// Gemeinsame Erzeugung temporärer Canvas-Flächen für Views und Meter.
+export function createCanvasSurface(width, height) {
+  const w = Math.max(1, Math.ceil(width));
+  const h = Math.max(1, Math.ceil(height));
+  if (typeof OffscreenCanvas === 'function') {
+    const canvas = new OffscreenCanvas(w, h);
+    const ctx = canvas.getContext('2d');
+    if (ctx) return { canvas, ctx, width: w, height: h };
+  }
+  if (typeof document !== 'undefined') {
+    const canvas = document.createElement('canvas');
+    canvas.width = w;
+    canvas.height = h;
+    const ctx = canvas.getContext('2d');
+    if (ctx) return { canvas, ctx, width: w, height: h };
+  }
+  return null;
+}