Add client-side signature overlay system documentation

Documented the new client-side signature overlay system for
EnvelopeViewer, replacing iText7 due to GPL license issues.
Outlined the signature data structure in C# and JavaScript,
and described the workflow steps for creating, applying,
and displaying signatures as visual overlays.

Added a "Future Enhancement Required" section suggesting
commercial PDF libraries or server-side stamping for future
improvements.

COPILOT_CONTEXT_EN.md

@@ -657,6 +657,73 @@ button.style.left/top
 
 ---
 
+## Signature Workflow in EnvelopeViewer — NEW Implementation (Session 13+)
+
+**IMPORTANT: iText7 NOT USED in EnvelopeViewer**
+- **Reason:** GPL license incompatibility (requires source code sharing)
+- **Alternative:** Client-side signature overlay system (HTML + Canvas API)
+- **Export:** Signatures are visual overlays only, NOT stamped on PDF bytes
+- **Future:** Consider PSPDFKit or commercial PDF library for actual PDF stamping
+
+### Signature Data Structure
+
+**Captured Signature (`_capturedSignature`):**
+```csharp
+record SignatureCapture(
+    string DataUrl,        // base64 PNG: "data:image/png;base64,iVBORw0KG..."
+    string FullName,       // Required: "Max Mustermann"
+    string Position,       // Optional: "Geschäftsführer"
+    string Place           // Required: "Berlin"
+);
+```
+
+**Applied Signature (per SignatureDto):**
+```javascript
+{
+    signatureId: 42,
+    dataUrl: "data:image/png;base64,iVBORw0KG...",
+    fullName: "Max Mustermann",
+    position: "Geschäftsführer",
+    place: "Berlin",
+    date: "15.01.2025",
+    x: 108,      // PDF POINTS (already converted from INCHES)
+    y: 144,      // PDF POINTS
+    width: 150,  // pixels
+    height: 60   // pixels
+}
+```
+
+### Workflow Steps
+
+**Step 1: Signature Creation Popup**
+- Reuse `DxPopup` from ReportViewer.razor
+- 3 tabs: Draw / Text / Image (using `receiver-signature.js`)
+- Fields: Full name, Position (optional), Place
+- Click "Speichern" ? `_capturedSignature` saved to state
+
+**Step 2: Apply Signature to Canvas**
+- User clicks "Unterschreiben" button on PDF
+- JavaScript creates HTML overlay (NOT Canvas drawing)
+- Overlay repositions on zoom/page change
+- **NO PDF byte modification** (GPL license issue)
+
+**Step 3: Visual Display Only**
+- Signatures shown as HTML `<div>` overlays
+- Positioned using absolute positioning
+- Persist in Blazor component state
+- Lost on page refresh (no server-side save)
+
+**Future Enhancement Required:**
+- Replace iText7 with commercial PDF library (e.g., PSPDFKit, Syncfusion)
+- Or use server-side stamping with Azure PDF Services
+- Or accept GPL license and open-source the stamping code
+
+---
+
+
+
+
+