Add signature input and PDF export functionality

Introduced a signature input feature in `ReportViewer.razor` to allow users to draw a signature on a canvas and embed it into reports before exporting as a signed PDF.

- Added a canvas (`receiver-signature-pad`) and buttons for clearing, applying, and exporting signatures.
- Injected `IJSRuntime` for JavaScript interop and added methods for signature handling (`ClearSignatureAsync`, `ApplySignatureAsync`, `ExportSignedPdfAsync`).
- Embedded the signature as an image and label in the report's bottom margin.
- Added `receiver-signature.js` to manage the signature pad, including drawing, clearing, and exporting the signature as a Base64 image.
- Updated `index.html` to include the new JavaScript file.
- Displayed validation messages for missing or invalid signatures.

EnvelopeGenerator.ReceiverUI/Pages/ReportViewer.razor

@@ -1,11 +1,33 @@
 @page "/reportviewer/"
+@using System.Drawing
+@using DevExpress.Drawing
+@using DevExpress.Utils
+@using DevExpress.XtraPrinting
+@using DevExpress.XtraPrinting.Drawing
 @using DevExpress.XtraReports.UI;
 @using EnvelopeGenerator.ReceiverUI.Services;
+@inject IJSRuntime JSRuntime
 @inject InMemoryReportStorageWebExtension ReportStorage
 
 <link href="_content/DevExpress.Blazor.Themes/blazing-berry.bs5.min.css" rel="stylesheet" />
 <link href="_content/DevExpress.Blazor.Reporting.Viewer/css/dx-blazor-reporting-components.bs5.css" rel="stylesheet" />
 
+<div class="card m-3">
+    <div class="card-body">
+        <h5 class="card-title">Unterschrift</h5>
+        <p class="card-text text-muted mb-2">Bitte fuegen Sie vor dem PDF-Export Ihre Unterschrift in das Feld unten ein.</p>
+        <canvas id="receiver-signature-pad" width="420" height="150" class="border rounded bg-white w-100" style="max-width: 420px; touch-action: none;"></canvas>
+        @if(!string.IsNullOrWhiteSpace(SignatureValidationMessage)) {
+            <div class="text-danger mt-2">@SignatureValidationMessage</div>
+        }
+        <div class="mt-3 d-flex gap-2 flex-wrap">
+            <button class="btn btn-outline-secondary" @onclick="ClearSignatureAsync">Unterschrift loeschen</button>
+            <button class="btn btn-primary" @onclick="ApplySignatureAsync">Unterschrift zum Bericht hinzufuegen</button>
+            <button class="btn btn-success" disabled="@(!SignatureApplied)" @onclick="ExportSignedPdfAsync">Signiertes PDF exportieren</button>
+        </div>
+    </div>
+</div>
+
 @if(Report is not null) {
     <DxReportViewer @ref="reportViewer" Report="Report" RootCssClasses="w-100 h-100" />
 }
@@ -13,6 +35,8 @@
 @code {
     DxReportViewer reportViewer;
     XtraReport? Report;
+    bool SignatureApplied;
+    string? SignatureValidationMessage;
 
     protected override async Task OnInitializedAsync() {
         Report = ReportStorage.TryGetReport("LargeDatasetReport", out var savedReport)
@@ -21,4 +45,100 @@
 
         await Task.CompletedTask;
     }
+
+    protected override async Task OnAfterRenderAsync(bool firstRender) {
+        if(firstRender)
+            await JSRuntime.InvokeVoidAsync("receiverSignature.initialize", "receiver-signature-pad");
+    }
+
+    async Task ClearSignatureAsync() {
+        await JSRuntime.InvokeVoidAsync("receiverSignature.clear", "receiver-signature-pad");
+        SignatureApplied = false;
+        SignatureValidationMessage = null;
+        Report = ReportStorage.TryGetReport("LargeDatasetReport", out var savedReport)
+            ? savedReport
+            : PredefinedReports.ReportsFactory.GetReport("LargeDatasetReport");
+
+        if(reportViewer is not null)
+            await reportViewer.OpenReportAsync(Report);
+    }
+
+    async Task ApplySignatureAsync() {
+        var signatureDataUrl = await JSRuntime.InvokeAsync<string?>("receiverSignature.getDataUrl", "receiver-signature-pad");
+
+        if(string.IsNullOrWhiteSpace(signatureDataUrl)) {
+            SignatureApplied = false;
+            SignatureValidationMessage = "Die Unterschrift ist fuer den PDF-Export erforderlich.";
+            return;
+        }
+
+        SignatureValidationMessage = null;
+        ApplySignatureToReport(signatureDataUrl);
+        SignatureApplied = true;
+
+        if(reportViewer is not null)
+            await reportViewer.OpenReportAsync(Report);
+    }
+
+    async Task ExportSignedPdfAsync() {
+        if(!SignatureApplied || Report is null) {
+            SignatureValidationMessage = "Bitte fuegen Sie die Unterschrift zuerst zum Bericht hinzu.";
+            return;
+        }
+
+        await reportViewer.ExportToAsync(ExportFormat.Pdf);
+    }
+
+    void ApplySignatureToReport(string signatureDataUrl) {
+        Report ??= ReportStorage.TryGetReport("LargeDatasetReport", out var savedReport)
+            ? savedReport
+            : PredefinedReports.ReportsFactory.GetReport("LargeDatasetReport");
+
+        AddSignature(Report, signatureDataUrl);
+    }
+
+    static void AddSignature(XtraReport report, string signatureDataUrl) {
+        var imageBytes = Convert.FromBase64String(signatureDataUrl[(signatureDataUrl.IndexOf(',') + 1)..]);
+        using var imageStream = new MemoryStream(imageBytes);
+        var imageSource = new ImageSource(DXImage.FromStream(imageStream));
+        var bottomMargin = report.Bands.OfType<BottomMarginBand>().FirstOrDefault();
+
+        if(bottomMargin is null) {
+            bottomMargin = new BottomMarginBand();
+            report.Bands.Add(bottomMargin);
+        }
+
+        bottomMargin.HeightF = Math.Max(bottomMargin.HeightF, 120F);
+        RemoveExistingSignature(bottomMargin);
+
+        var signatureLabel = new XRLabel {
+            Name = "receiverSignatureLabel",
+            Text = $"Empfaengerunterschrift - {DateTime.Now:g}",
+            BoundsF = new RectangleF(390F, 6F, 230F, 18F),
+            Font = new DXFont("Open Sans", 8F, DXFontStyle.Bold),
+            ForeColor = System.Drawing.Color.FromArgb(73, 80, 87),
+            TextAlignment = TextAlignment.MiddleLeft
+        };
+
+        var signature = new XRPictureBox {
+            Name = "receiverSignatureImage",
+            ImageSource = imageSource,
+            BoundsF = new RectangleF(390F, 28F, 230F, 70F),
+            Sizing = ImageSizeMode.ZoomImage,
+            Borders = BorderSide.Bottom,
+            BorderColor = System.Drawing.Color.FromArgb(73, 80, 87)
+        };
+
+        bottomMargin.Controls.AddRange(new XRControl[] { signatureLabel, signature });
+    }
+
+    static void RemoveExistingSignature(BottomMarginBand bottomMargin) {
+        var controls = bottomMargin.Controls
+            .Cast<XRControl>()
+            .Where(control => control.Name is "receiverSignatureLabel" or "receiverSignatureImage")
+            .ToArray();
+
+        foreach(var control in controls)
+            bottomMargin.Controls.Remove(control);
+    }
 }