Add interactive signature buttons to PDF viewer

Implemented a new feature to render clickable "Sign" buttons on the PDF canvas at signature field positions fetched from the database.

- Updated `EnvelopeViewer.razor` to fetch signature data, convert coordinates from inches to points, and invoke JavaScript for rendering.
- Added `renderSignatureButtons` and `clearSignatureButtons` functions in `pdf-viewer.js` to dynamically create and position buttons based on page and zoom level.
- Modified HTML to include a new `#pdf-signature-layer` overlay for buttons.
- Styled buttons with a purple gradient, hover/active effects, and focus outlines for accessibility.
- Defined rendering triggers for initial load, page changes, and zoom changes.
- Documented coordinate conversion flow from inches to points to pixels for accurate positioning.
- Enhanced accessibility with `tabindex="0"`, focus outlines, and semantic `<button>` elements.

COPILOT_CONTEXT_EN.md

@@ -419,6 +419,14 @@ Our use case is **visual/image stamping** at specific page coordinates
 | **11** | **2025-01-XX** | **Made zoom step configurable (buttons, Ctrl+Wheel, slider use same step)** |
 | **11** | **2025-01-XX** | **Fixed thumbnail canvas alignment (object-fit: contain)** |
 | **11** | **2025-01-XX** | **Fixed thumbnail re-rendering on sidebar toggle** |
+| **12** | **2025-01-XX** | **Fixed coordinate system documentation: Database stores INCHES (not DX units)** |
+| **12** | **2025-01-XX** | **Updated XML documentation in SignatureDto, AnnotationDto, AnnotationCreateDto** |
+| **13** | **2025-01-XX** | **Added SenderAppType enum to SignatureDto for Legacy/Blazor app differentiation** |
+| **13** | **2025-01-XX** | **Implemented signature overlay rendering in PDF.js viewer (INCHES ? normalized ? pixels)** |
+| **13** | **2025-01-XX** | **Added automatic overlay re-rendering on page change, zoom, and initial load** |
+| **13** | **2025-01-XX** | **Added clickable signature buttons on PDF canvas (Sign/Unterschreiben)** |
+| **13** | **2025-01-XX** | **Signature buttons: 150px×60px, purple gradient, pen icon, hover effects** |
+| **13** | **2025-01-XX** | **JavaScript: `pdfViewer.renderSignatureButtons()` and `pdfViewer.clearSignatureButtons()`** |
 
 ---
 
@@ -503,4 +511,154 @@ Our use case is **visual/image stamping** at specific page coordinates
 - **Mobile (?768px)**: Flex column, thumbnails on top
 - **Thumbnail toggle**: Controlled by `@if (_showThumbnails)` in Razor markup
 
+---
+
+## Signature Buttons in EnvelopeViewer — Interactive Overlay System
+
+**Purpose:** Render clickable "Unterschreiben" (Sign) buttons on PDF canvas at signature field positions fetched from database.
+
+### Architecture
+
+**Blazor Component (`EnvelopeViewer.razor`):**
+```csharp
+IReadOnlyList<SignatureDto> _signatures = [];
+
+protected override async Task OnInitializedAsync() {
+    var signatures = await SignatureService.GetAsync(EnvelopeKey);
+    _signatures = signatures.Convert(UnitOfLength.Point);  // INCHES ? POINTS
+}
+
+async Task RenderSignatureButtonsAsync() {
+    await JSRuntime.InvokeVoidAsync("pdfViewer.renderSignatureButtons", 
+        _signatures, _currentPage, _dotNetRef);
+}
+
+[JSInvokable]
+public void OnSignatureButtonClick(int signatureId) {
+    Console.WriteLine($"Signature #{signatureId} signed");
+}
+```
+
+**JavaScript (`pdf-viewer.js`):**
+```javascript
+renderSignatureButtons(signatures, currentPageNum, dotNetRef) {
+    this.clearSignatureButtons();  // Remove old buttons
+    
+    const pageSignatures = signatures.filter(sig => sig.page === currentPageNum);
+    const signatureLayer = document.getElementById('pdf-signature-layer');
+    
+    pageSignatures.forEach(sig => {
+        // Convert POINTS to display pixels
+        const xPx = sig.x * this.scale;  // sig.x already in PDF POINTS
+        const yPx = sig.y * this.scale;
+        
+        const button = document.createElement('button');
+        button.className = 'signature-button';
+        button.style.left = `${xPx}px`;
+        button.style.top = `${yPx}px`;
+        button.style.width = '150px';
+        button.style.height = '60px';
+        
+        // German text + pen icon
+        button.innerHTML = `
+            <div>Unterschreiben</div>
+            <svg>...</svg>
+        `;
+        
+        button.addEventListener('click', () => {
+            this.dotNetReference.invokeMethodAsync('OnSignatureButtonClick', sig.id);
+        });
+        
+        signatureLayer.appendChild(button);
+        this.signatureButtons.push(button);
+    });
+}
+
+clearSignatureButtons() {
+    this.signatureButtons.forEach(btn => btn.parentNode?.removeChild(btn));
+    this.signatureButtons = [];
+}
+```
+
+**HTML Structure:**
+```html
+<div class="pdf-page-container">
+    <canvas id="pdf-canvas"></canvas>
+    <div id="pdf-text-layer"></div>
+    <div id="pdf-signature-layer"></div>  <!-- NEW -->
+</div>
+```
+
+**CSS (`envelope-viewer.css`):**
+```css
+.pdf-signature-layer {
+    position: absolute;
+    left: 0; top: 0; right: 0; bottom: 0;
+    overflow: visible;
+    pointer-events: none;  /* Pass clicks through to canvas */
+    z-index: 20;
+}
+
+.signature-button {
+    pointer-events: auto;  /* Re-enable for buttons */
+    position: absolute;
+    width: 150px; height: 60px;
+    background: linear-gradient(135deg, #4F46E5 0%, #4338CA 100%);
+    color: white;
+    border: none;
+    border-radius: 8px;
+    cursor: pointer;
+    /* Hover: scale(1.05), shadow, darker gradient */
+}
+```
+
+### Rendering Triggers
+
+1. **Initial Load:** After PDF renders (`OnAfterRenderAsync`)
+2. **Page Change:** `NextPage()`, `PreviousPage()`, `GoToPageFromThumbnail()`
+3. **Zoom Change:** `OnZoomChanged()` (re-calculates pixel positions)
+
+### Coordinate Conversion Flow
+
+```
+Database (INCHES)
+  ? SignatureService.GetAsync()
+SignatureDto.X/Y (INCHES)
+  ? .Convert(UnitOfLength.Point)
+SignatureDto.X/Y (PDF POINTS, × 72)
+  ? JavaScript
+Display Pixels (× this.scale)
+  ? CSS
+button.style.left/top
+```
+
+**Example Calculation:**
+- Database: `X = 1.5 inches, Y = 2.0 inches`
+- After conversion: `X = 108 points, Y = 144 points` (× 72)
+- At scale 1.5: `X = 162px, Y = 216px`
+- Button positioned at `left: 162px, top: 216px`
+
+### Button Design
+
+**Visual Spec:**
+- **Size:** 150px × 60px
+- **Background:** Purple gradient `#4F46E5` ? `#4338CA` (hover: darker)
+- **Text:** "Unterschreiben" (18px, bold, white)
+- **Icon:** Pen SVG (24px white)
+- **Effects:**
+  - Hover: `scale(1.05)` + shadow `0 4px 12px rgba(79, 70, 229, 0.4)`
+  - Active: `scale(0.98)`
+  - Focus: `2px solid #7e22ce` outline
+
+**Accessibility:**
+- `tabindex="0"` for keyboard navigation
+- Focus outline for keyboard users
+- Click handler with semantic button element
+
+---
+
+
+
+
+