feat: add label preview functionality to barcode templates with backend rendering and UI modal

paperless-backend/src/label-print-agent/label-print-agent.controller.ts

@@ -24,6 +24,19 @@ import { LabelPrintAgentService } from './label-print-agent.service';
 export class LabelPrintAgentController {
   constructor(private readonly service: LabelPrintAgentService) {}
 
+  @Post('preview')
+  @HttpCode(HttpStatus.OK)
+  @RequirePermissions(Permission.VIEW_SCANNER)
+  async preview(
+    @Body() body: { templateId: number; fieldValues?: Record<string, string> },
+    @Res({ passthrough: true }) res: Response,
+  ): Promise<StreamableFile> {
+    const buf = await this.service.renderPreview(body.templateId, body.fieldValues ?? {});
+    const { Readable } = await import('stream');
+    res.setHeader('Content-Type', 'image/png');
+    return new StreamableFile(Readable.from(buf));
+  }
+
   // Manuell einen Job anlegen (Frontend → Backend)
   @Post('jobs')
   @HttpCode(HttpStatus.CREATED)

paperless-backend/src/label-print-agent/label-print-agent.service.ts

@@ -155,6 +155,39 @@ export class LabelPrintAgentService {
     await this.callUrl('RELEASE', job);
   }
 
+  async renderPreview(templateId: number, fieldValues: Record<string, string>): Promise<Buffer> {
+    const template = await this.templateRepo.findOne({ where: { Id: templateId } });
+    if (!template) throw new NotFoundException('Template nicht gefunden');
+    if (!template.LabelLayout?.length) throw new BadRequestException('Kein Layout definiert');
+
+    const vars: Record<string, string> = { ...fieldValues };
+    for (const field of template.LabelInputFields ?? []) {
+      if (field.type === 'date' && fieldValues[field.name]) {
+        const parts = fieldValues[field.name].split('-');
+        if (parts.length === 3) {
+          vars[`${field.name}.year`] = parts[0];
+          vars[`${field.name}.month`] = parts[1];
+          vars[`${field.name}.day`] = parts[2];
+        }
+      }
+    }
+    if (template.LabelGetUrl) {
+      const url = applyVars(template.LabelGetUrl, vars);
+      try {
+        vars['number'] = (await (await fetch(url)).text()).trim();
+      } catch (err: any) {
+        this.logger.warn(`GET-URL fehlgeschlagen (${url}): ${err.message}`);
+      }
+    }
+
+    return this.renderer.render(
+      template.LabelLayout,
+      template.LabelWidthMm ?? 57,
+      template.LabelHeightMm ?? 32,
+      vars,
+    );
+  }
+
   private async callUrl(type: 'PRINTED' | 'RELEASE', job: LabelPrintJob): Promise<void> {
     const template = job.BarcodeTemplateId
       ? await this.templateRepo.findOne({ where: { Id: job.BarcodeTemplateId } })