Add EnvelopeViewer for read-only PDF viewing with PDF.js

Introduced a new EnvelopeViewer component (`EnvelopeViewer.razor`)
to replace the legacy ReportViewer for non-signing workflows.
The new viewer is based on PDF.js and provides a modern,
lightweight, and responsive user experience.

Added `pdf-viewer.js` to handle PDF rendering, zoom, pagination,
and mouse wheel control, integrated with Blazor via JSInterop.
Externalized styles to `envelope-viewer.css` with a modern
glassmorphism design.

Enhanced user experience with unlimited zoom, global mouse
wheel zoom (`Ctrl+Wheel`), and responsive design. Fixed issues
like scroll behavior and canvas size restrictions.

Updated `COPILOT_CONTEXT_EN.md` to document the new
EnvelopeViewer, its architecture, and its advantages over the
legacy ReportViewer. Added external dependencies for PDF.js
via CDN and updated project history to reflect these changes.

COPILOT_CONTEXT_EN.md

@@ -25,8 +25,11 @@ A digital document signing system. Senders upload PDFs and place signature annot
 
 | File | Purpose |
 |---|---|
-| `ReceiverUI/Pages/ReportViewer.razor` | Main receiver page. All signing logic. |
+| `ReceiverUI/Pages/EnvelopeViewer.razor` | **NEW** PDF.js-based viewer (`/envelope/{key}`). Replaces ReportViewer.razor. Simple read-only PDF viewing with zoom/navigation. |
+| `ReceiverUI/Pages/ReportViewer.razor` | **LEGACY** DevExpress-based signing page (`/receiver/{key}`). Still used for signature workflow. Will be deprecated. |
+| `ReceiverUI/wwwroot/js/pdf-viewer.js` | **NEW** PDF.js wrapper: rendering, zoom, pagination, mouse wheel control. |
 | `ReceiverUI/wwwroot/js/receiver-signature.js` | JS: checkbox overlay, signature pad (draw/type/image). |
+| `ReceiverUI/wwwroot/css/envelope-viewer.css` | **NEW** Styles for EnvelopeViewer.razor (external CSS, not inline). |
 | `ReceiverUI/wwwroot/fake-data/annotations.json` | Dev-mode fake annotations (YARP proxy target). |
 | `ReceiverUI/Models/AnnotationDto.cs` | Annotation position model. All properties non-nullable. |
 | `ReceiverUI/Services/AnnotationService.cs` | Fetches `List<AnnotationDto>` from API or fake-data. |
@@ -55,7 +58,109 @@ Conversions:
 
 ---
 
-## ReceiverUI Signing Flow (ReportViewer.razor)
+## EnvelopeViewer (NEW) — PDF.js Read-Only Viewer
+
+**Route:** `/envelope/{EnvelopeKey}`  
+**Purpose:** Simple, modern PDF viewing without signing functionality.  
+**Technology:** PDF.js 3.11.174 + custom JavaScript wrapper
+
+### Architecture
+
+**Blazor Component (`EnvelopeViewer.razor`):**
+- Fetches PDF via `DocumentService.GetDocumentAsync(EnvelopeKey)`
+- Converts to base64 data URL: `data:application/pdf;base64,{base64}`
+- Initializes PDF.js viewer via JSInterop with `DotNetObjectReference` for callbacks
+- Displays controls: Zoom In/Out, Page Navigation, Zoom percentage
+- CSS externalized to `envelope-viewer.css`
+
+**JavaScript (`pdf-viewer.js`):**
+```javascript
+window.pdfViewer = {
+    pdfDoc, canvas, ctx, scale, currentRenderTask,
+    dotNetReference, wheelEventAttached,
+    
+    initialize(canvasId, pdfDataUrl, dotNetRef),
+    renderPage(num),
+    attachWheelEvent(),  // Global Ctrl+Wheel zoom
+    zoomIn(), zoomOut(),
+    nextPage(), previousPage(),
+    dispose()
+}
+```
+
+**CSS (`envelope-viewer.css`):**
+- `.envelope-viewer-layout`: Full-height gradient background
+- `.envelope-action-bar`: Top bar with logo, title, controls (sticky)
+- `.pdf-frame`: Fixed-size white container (`calc(100vh - 200px)` × 90% width, max 1200px)
+- `.pdf-canvas`: `display: inline-block`, unlimited zoom, scrollable when exceeds frame
+- Modern glassmorphism design with gradients and shadows
+
+### Features
+
+1. **Unlimited Zoom:**
+   - Canvas size not restricted by `max-width`
+   - Frame stays fixed, scroll bars appear automatically
+   - `text-align: center` for small sizes, full scroll for zoomed views
+
+2. **Global Mouse Wheel Zoom:**
+   - Event listener on `document.body` (works anywhere on page)
+   - `Ctrl + Mouse Wheel` triggers `zoomIn()`/`zoomOut()`
+   - Calls `dotNetReference.invokeMethodAsync('OnZoomChanged', scale)` to update Blazor UI
+   - `{ passive: false }` to enable `preventDefault()`
+
+3. **Render Task Cancellation:**
+   - Stores `currentRenderTask` to cancel previous render if new one starts
+   - Catches `RenderingCancelledException` to avoid console errors
+   - Queue system (`pageNumPending`) for rapid page changes
+
+4. **Responsive Design:**
+   - Desktop: 90% width, 1200px max
+   - Mobile: 95% width, adjusted heights
+   - Adaptive padding and font sizes
+
+### Flow
+
+1. **Component Load:**
+   ```csharp
+   OnInitializedAsync():
+     - Fetch PDF bytes
+     - Convert to base64 data URL
+     - Set _isLoading = false
+   
+   OnAfterRenderAsync():
+     - Create DotNetObjectReference
+     - JSRuntime.InvokeAsync("pdfViewer.initialize", canvasId, pdfDataUrl, dotNetRef)
+     - Update _totalPages, _currentPage, _pdfLoaded
+   ```
+
+2. **User Interaction:**
+   - Button clicks ? `ZoomIn()`/`ZoomOut()` ? `JSRuntime.InvokeVoidAsync("pdfViewer.zoomIn")`
+   - Ctrl+Wheel ? JS `attachWheelEvent()` ? `dotNetRef.invokeMethodAsync('OnZoomChanged')`
+   - Page buttons ? `NextPage()`/`PreviousPage()` ? `JSRuntime.InvokeAsync("pdfViewer.nextPage")`
+
+3. **Cleanup:**
+   ```csharp
+   DisposeAsync():
+     - JSRuntime.InvokeVoidAsync("pdfViewer.dispose")
+     - _dotNetRef?.Dispose()
+   ```
+
+### Key Differences from ReportViewer
+
+| Feature | EnvelopeViewer (NEW) | ReportViewer (LEGACY) |
+|---------|----------------------|------------------------|
+| Technology | PDF.js + Canvas | DevExpress XtraReports |
+| Route | `/envelope/{key}` | `/receiver/{key}` |
+| Purpose | Read-only viewing | Signature workflow |
+| Dependencies | PDF.js CDN | DevExpress NuGet packages |
+| Zoom | Unlimited, smooth | Report viewer default |
+| Mouse Wheel | Custom Ctrl+Wheel | Browser default |
+| File Size | Minimal (JS + CSS) | Heavy (DX libs) |
+| Maintenance | Simple, standard web | Complex, vendor-specific |
+
+---
+
+## ReceiverUI Signing Flow (ReportViewer.razor — LEGACY)
 
 ### On Load (`OnInitializedAsync`)
 1. `AuthService.CheckEnvelopeAccessAsync` ? redirect to login if unauthorized
@@ -137,12 +242,16 @@ return report;
 
 | Package | Version | Purpose |
 |---|---|---|
-| `DevExpress.Blazor.Reporting.Viewer` | 25.2.3 | DxReportViewer component |
-| `DevExpress.Blazor.PdfViewer` | 25.2.3 | PDF viewer |
+| `DevExpress.Blazor.Reporting.Viewer` | 25.2.3 | DxReportViewer (LEGACY, used in ReportViewer.razor) |
+| `DevExpress.Blazor.PdfViewer` | 25.2.3 | PDF viewer (not used in EnvelopeViewer) |
 | `DevExpress.Drawing.Skia` | 25.2.3 | Drawing backend |
 | `itext` | 8.0.5 | PDF stamping (iText7) |
 | `SkiaSharp.*` | 3.119.1 | WASM native rendering |
 
+**External CDN (EnvelopeViewer):**
+- PDF.js 3.11.174 (via `https://cdnjs.cloudflare.com/ajax/libs/pdf.js/3.11.174/pdf.min.js`)
+- PDF.js Worker (`pdf.worker.min.js`)
+
 ---
 
 ## Mistakes History — Do NOT Repeat
@@ -155,6 +264,9 @@ return report;
 | `ctrl.Report?.PrintingSystem` | `PrintingSystem` not on `XtraReportBase` in WASM |
 | Adding stamp endpoint to `DocumentController` | Not needed; stamping is done client-side in ReceiverUI |
 | iText7 via API (server-side) | Unnecessary; iText7 runs fine in WASM directly |
+| **PDF.js: `display: flex` on `.pdf-frame`** | **Prevents left-edge scroll when canvas exceeds container** |
+| **PDF.js: `max-width: 100%` on canvas** | **Limits zoom; user expects unlimited zoom capability** |
+| **Mouse wheel on `.pdf-frame` only** | **Only works when mouse over PDF; should work anywhere on page** |
 
 ---
 
@@ -180,3 +292,10 @@ Our use case is **visual/image stamping** at specific page coordinates
 | 10 | — | Investigated DevExpress article — not applicable to our case |
 | 10 | — | Added iText7 to ReceiverUI; implemented `StampSignaturesOnPdf` — ? deterministic coordinates, no page count side effects |
 | 10 | — | Split COPILOT_CONTEXT.md into COPILOT_CONTEXT_EN.md and COPILOT_CONTEXT_TR.md |
+| **11** | **2025-01-XX** | **Created EnvelopeViewer.razor (`/envelope/{key}`) with PDF.js 3.11.174** |
+| **11** | **2025-01-XX** | **Implemented `pdf-viewer.js`: canvas rendering, zoom, pagination, render task cancellation** |
+| **11** | **2025-01-XX** | **Externalized CSS to `envelope-viewer.css`: modern glassmorphism design** |
+| **11** | **2025-01-XX** | **Fixed scroll issues: removed `display: flex`, used `text-align: center` + `inline-block`** |
+| **11** | **2025-01-XX** | **Removed canvas `max-width` restriction for unlimited zoom** |
+| **11** | **2025-01-XX** | **Added global mouse wheel zoom: `Ctrl+Wheel` on `document.body`, JSInterop callback to Blazor** |
+| **11** | **2025-01-XX** | **Updated COPILOT_CONTEXT_EN.md: EnvelopeViewer replaces ReportViewer for read-only viewing** |