Refactor EnvelopeReceiverPage to server-side logic

Updated the document signing system to use a unified Blazor Auto (Server+WASM hybrid) frontend. Replaced client-side API calls with server-side authentication and data loading via `EnvelopeReceiverAuthorizationService` and `EnvelopeReceiverPageDataService`.

- Updated `/envelope/{key}` route to use MediatR for data loading.
- Integrated PDF.js 3.11.174 for rendering with configurable quality.
- Removed iText7 dependency due to GPL license issues.
- Introduced per-envelope cookies for receiver authentication.
- Cached signatures now loaded from distributed cache.
- Replaced redundant client-side API calls with server-side logic.
- Improved security and performance with server-side authorization.

These changes streamline the workflow, enhance security, and align the system with modern Blazor Server practices.

COPILOT_CONTEXT.md

@@ -5,6 +5,11 @@ Digital document signing system with **unified Blazor Auto (Server+WASM hybrid)
 
 **Primary Libraries:** DevExpress + PDF.js (PSPDFKit removed)
 
+**Receiver Architecture:**
+- Receiver authentication for `EnvelopeReceiverPage.razor` is now validated server-side.
+- Receiver page data is loaded directly via MediatR and distributed cache, not through the page's own API calls.
+- PDF rendering in `EnvelopeReceiverPage.razor` is PDF.js-based, while `DxPdfViewer` remains the SSR-native viewer target.
+
 ---
 
 ## Migration Notice
@@ -203,7 +208,7 @@ Client ? WebUI:XXXX (Blazor Auto)
 ### Server-Side Pages (WebUI)
 | File | Route | Purpose |
 |---|---|---|
-| `WebUI/Components/Pages/EnvelopeReceiverPage.razor` | `/envelope/{key}` | Receiver PDF viewer & signing (PDF.js). |
+| `WebUI/Components/Pages/EnvelopeReceiverPage.razor` | `/envelope/{key}` | Receiver PDF viewer & signing page. Uses Interactive Server, server-side auth/data loading, and currently renders with PDF.js overlay logic. |
 | `WebUI/Components/Pages/EnvelopeReceiverPage_DxPdfViewer.razor` | `/envelope/DxPdfViewer` | DevExpress PDF Viewer (test page). |
 | `WebUI/Components/Pages/EnvelopeReceiverPage_DxReportViewer.razor` | `/envelope/{key}/DxReportViewer` | DevExpress Report Viewer. |
 | `WebUI/Components/Pages/EnvelopeReceiverPage_embed.razor` | `/envelope/Embed` | Embedded PDF viewer (iframe). |
@@ -214,6 +219,8 @@ Client ? WebUI:XXXX (Blazor Auto)
 | `WebUI.Client/Services/AuthService.cs` | Receiver + Sender authentication. |
 | `WebUI.Client/Services/SignatureCacheService.cs` | Signature caching (Redis/SQL). |
 | `WebUI.Client/Services/DocumentService.cs` | PDF document retrieval. |
+| `WebUI/Services/EnvelopeReceiverAuthorizationService.cs` | Server-side receiver authorization for `EnvelopeReceiverPage.razor` using per-envelope cookie/JWT validation. |
+| `WebUI/Services/EnvelopeReceiverPageDataService.cs` | Server-side document/signature/receiver data loading via MediatR and distributed cache. |
 | `WebUI/wwwroot/js/pdf-viewer.js` | PDF.js wrapper (zoom, pagination, thumbnails). |
 | `WebUI/wwwroot/js/receiver-signature.js` | Signature pad (draw/type/image). |
 | `WebUI/wwwroot/css/envelope-viewer.css` | EnvelopeViewer styles. |
@@ -253,9 +260,16 @@ Client ? WebUI:XXXX (Blazor Auto)
 ## EnvelopeReceiver — PDF.js Viewer & Signing
 
 **Route:** `/envelope/{EnvelopeKey}`  
-**Tech:** PDF.js 3.11.174 + Blazor Server (`@rendermode InteractiveServer`) + configurable quality  
+**Tech:** PDF.js 3.11.174 + Blazor Server (`@rendermode InteractiveServer`) + server-side auth/data loading + configurable quality  
 **File:** `WebUI/Components/Pages/EnvelopeReceiverPage.razor`
 
+### Current Server-Side Loading Model
+
+- Authorization is performed inside the server project via `EnvelopeReceiverAuthorizationService`.
+- The page no longer relies on `GET /api/auth/check/envelope/{EnvelopeKey}` for its own access check.
+- Document bytes, receiver data, and signature placeholders are loaded directly through MediatR using `EnvelopeReceiverPageDataService`.
+- Cached signatures are loaded from distributed cache directly in the server project.
+
 ### Key Features
 1. HiDPI/Retina support (4x quality)
 2. Configurable quality (`appsettings.json`)
@@ -300,7 +314,9 @@ window.pdfViewer = {
 ### Workflow Steps
 
 1. **Page Load:**
-   - Check `SignatureCacheService` for cached signature
+   - Validate receiver access server-side using the per-envelope auth cookie
+   - Load document, receiver, and signature data directly through MediatR
+   - Check distributed cache for cached signature
    - If cached ? skip popup, load signature
    - If not ? show automatic popup (mandatory)
 
@@ -325,6 +341,12 @@ window.pdfViewer = {
    - Zoom/Page change ? recalculate button positions
    - Session state: `_capturedSignature` (lost on refresh)
 
+### Authentication Notes
+
+- Receiver cookies are stored per envelope: `AuthTokenSignFLOWReceiver.{envelopeKey}`.
+- `EnvelopeReceiverPage.razor` uses server-side receiver authorization logic instead of calling its own auth check API endpoint.
+- The server-side auth flow must remain compatible with `AuthScheme.Receiver` and `AuthPolicy.Receiver`.
+
 ### Data Model
 **File:** `WebUI.Client/Models/SignatureCaptureDto.cs`