feat: Etiketten-Seitenlayout auf dem A4-Bogen frei waehlbar

- LabelTemplate bekommt columns/rows (optional, leer = automatisch
  passend), plus konfigurierbare Raender/Abstaende - damit ein Etikett
  exakt auf das vorgedruckte Raster eines physischen Etikettenbogens
  (z.B. Avery) gelegt werden kann statt nur automatisch berechnet
- Startposition beim Drucken waehlbar, fuer bereits angebrochene Boegen
  (erste Seite startet ab der gewaehlten Position, Folgeseiten wieder
  von vorne)

ApiCore-Backend/src/label-templates/label-templates.service.ts

@@ -13,12 +13,21 @@ import {
 } from './label-element.types';
 
 const MM_TO_PT = 72 / 25.4;
-const PAGE_MARGIN_MM = 5;
-const LABEL_GAP_MM = 2;
 const A4_WIDTH_MM = 210;
 const A4_HEIGHT_MM = 297;
 const QR_SOURCE_SIZE_PX = 400;
 
+interface SheetLayout {
+  widthMm: number;
+  heightMm: number;
+  columns: number | null;
+  rows: number | null;
+  marginTopMm: number;
+  marginLeftMm: number;
+  gapXMm: number;
+  gapYMm: number;
+}
+
 function formatDate(date: Date): string {
   return date.toLocaleDateString('de-DE');
 }
@@ -151,6 +160,7 @@ export class LabelTemplatesService {
   async renderLabelsPdf(
     templateId: string,
     bottlingBatchId: string,
+    startPosition = 1,
   ): Promise<Buffer> {
     const template = await this.findById(templateId);
     const bottlingBatch = await this.prisma.bottlingBatch.findUnique({
@@ -193,19 +203,28 @@ export class LabelTemplatesService {
 
     return this.renderPdf(
       elements,
-      template.widthMm,
-      template.heightMm,
+      {
+        widthMm: template.widthMm,
+        heightMm: template.heightMm,
+        columns: template.columns,
+        rows: template.rows,
+        marginTopMm: template.marginTopMm,
+        marginLeftMm: template.marginLeftMm,
+        gapXMm: template.gapXMm,
+        gapYMm: template.gapYMm,
+      },
       bottlingBatch.jarCount,
       qrCodeBuffer,
+      startPosition,
     );
   }
 
   private renderPdf(
     elements: LabelElement[],
-    widthMm: number,
-    heightMm: number,
+    layout: SheetLayout,
     copies: number,
     qrCodeBuffer: Buffer | null,
+    startPosition: number,
   ): Promise<Buffer> {
     return new Promise((resolve, reject) => {
       const doc = new PDFDocument({ size: 'A4', margin: 0 });
@@ -214,27 +233,34 @@ export class LabelTemplatesService {
       doc.on('end', () => resolve(Buffer.concat(chunks)));
       doc.on('error', (error: Error) => reject(error));
 
-      const usableWidthMm = A4_WIDTH_MM - 2 * PAGE_MARGIN_MM;
-      const usableHeightMm = A4_HEIGHT_MM - 2 * PAGE_MARGIN_MM;
-      const columns = Math.max(
-        1,
-        Math.floor((usableWidthMm + LABEL_GAP_MM) / (widthMm + LABEL_GAP_MM)),
-      );
-      const rows = Math.max(
-        1,
-        Math.floor((usableHeightMm + LABEL_GAP_MM) / (heightMm + LABEL_GAP_MM)),
-      );
+      const { widthMm, heightMm, marginTopMm, marginLeftMm, gapXMm, gapYMm } =
+        layout;
+      const usableWidthMm = A4_WIDTH_MM - 2 * marginLeftMm;
+      const usableHeightMm = A4_HEIGHT_MM - 2 * marginTopMm;
+      const columns =
+        layout.columns ??
+        Math.max(1, Math.floor((usableWidthMm + gapXMm) / (widthMm + gapXMm)));
+      const rows =
+        layout.rows ??
+        Math.max(
+          1,
+          Math.floor((usableHeightMm + gapYMm) / (heightMm + gapYMm)),
+        );
       const perPage = columns * rows;
+      // startPosition only skips ahead on the first sheet (partially-used
+      // physical sheet, e.g. a few labels already peeled off) - every sheet
+      // after that always starts fresh at the top-left slot.
+      const skip = Math.min(Math.max(0, startPosition - 1), perPage - 1);
 
       for (let index = 0; index < copies; index += 1) {
-        const onPage = index % perPage;
-        if (index > 0 && onPage === 0) {
+        const slotOnPage = (skip + index) % perPage;
+        if (index > 0 && slotOnPage === 0) {
           doc.addPage({ size: 'A4', margin: 0 });
         }
-        const column = onPage % columns;
-        const row = Math.floor(onPage / columns);
-        const xMm = PAGE_MARGIN_MM + column * (widthMm + LABEL_GAP_MM);
-        const yMm = PAGE_MARGIN_MM + row * (heightMm + LABEL_GAP_MM);
+        const column = slotOnPage % columns;
+        const row = Math.floor(slotOnPage / columns);
+        const xMm = marginLeftMm + column * (widthMm + gapXMm);
+        const yMm = marginTopMm + row * (heightMm + gapYMm);
         const originX = xMm * MM_TO_PT;
         const originY = yMm * MM_TO_PT;
         const w = widthMm * MM_TO_PT;