task(E2): arc-row tool for curved seating around stage

frontend/src/services/seatingService.ts

@@ -280,6 +280,47 @@ export class SeatingService {
     return elements;
   }
 
+  /** Create an arc row (curved seating around a center point) — Task E2 */
+  createArcRow(
+    centerX: number,
+    centerY: number,
+    radius: number,
+    startAngleDeg: number,
+    endAngleDeg: number,
+    layerId: string,
+    spacing = 50,
+  ): CADElement[] {
+    const arcLen = Math.abs(endAngleDeg - startAngleDeg) * Math.PI / 180 * radius;
+    const count = Math.max(2, Math.floor(arcLen / spacing));
+    const rowId = `arcrow_${Date.now().toString(36)}_${Math.random().toString(36).slice(2, 8)}`;
+    const elements: CADElement[] = [];
+    for (let i = 0; i <= count; i++) {
+      const angle = startAngleDeg + (endAngleDeg - startAngleDeg) * (i / count);
+      const rad = angle * Math.PI / 180;
+      const px = centerX + radius * Math.cos(rad);
+      const py = centerY + radius * Math.sin(rad);
+      elements.push({
+        id: this.generateId(),
+        type: 'chair',
+        layerId,
+        x: px,
+        y: py,
+        width: 45,
+        height: 45,
+        properties: {
+          rotation: angle + 90,
+          fill: '#4a90d9',
+          backrestColor: '#3a7ac9',
+          outlineColor: '#2a5a99',
+          seatType: 'standard',
+          rowIndex: i,
+          rowId,
+        },
+      });
+    }
+    return elements;
+  }
+
   /** Create a table element */
   createTable(x: number, y: number, layerId: string, config: Partial<TableConfig> = {}): CADElement {
     const cfg = { ...DEFAULT_TABLE, ...config };