task(CAD-6): background rendering fix - bgConfig was never wired to render engine (import worked but never drew), canvas area setOptions + pixi sprite implementation

frontend/src/components/CanvasArea.tsx

@@ -378,6 +378,19 @@ const CanvasArea: React.FC<CanvasAreaProps> = ({
     renderEngine.setOptions({ showOrtho: orthoEnabled });
     renderEngine.setOptions({ gridSize, unit, scaleFactor, showGridLabels: true });
     renderEngine.setOptions({ canvasBgColor, gridColor, rulerEnabled });
+
+    // ── Task CAD-6: Hintergrund-Grundriss (bgConfig) an RenderEngine geben ──
+    // Vorher wurde bgConfig als Prop uebergeben aber NIE genutzt — der
+    // importierte Grundriss wurde nicht gezeichnet.
+    renderEngine.setOptions({
+      backgroundSrc: bgConfig?.visible ? (bgConfig.src || undefined) : undefined,
+      backgroundScale: bgConfig?.scale ?? 1,
+      backgroundOffsetX: bgConfig?.offsetX ?? 0,
+      backgroundOffsetY: bgConfig?.offsetY ?? 0,
+      backgroundRotation: ((bgConfig?.rotation ?? 0) * Math.PI) / 180,
+      backgroundOpacity: bgConfig?.opacity ?? 0.5,
+    });
+    renderEngine.render();
     interaction.setSnapEnabled(snapEnabled);
     interaction.setOrthoEnabled(orthoEnabled);
     interaction.setPolarEnabled(polarEnabled);
@@ -385,7 +398,7 @@ const CanvasArea: React.FC<CanvasAreaProps> = ({
     interaction.setGridSize(gridSize);
     renderEngine.render();
   // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [gridEnabled, snapEnabled, orthoEnabled, polarEnabled, unit, gridSize, scaleFactor, canvasBgColor, gridColor, rulerEnabled]);
+  }, [gridEnabled, snapEnabled, orthoEnabled, polarEnabled, unit, gridSize, scaleFactor, canvasBgColor, gridColor, rulerEnabled, bgConfig]);
 
   // Sync active tool
   useEffect(() => {