Unify architecture with Blazor WASM and YARP proxy

Refactored the deployment architecture to include two distinct
presentation projects: `EnvelopeGenerator.API` (ASP.NET Core Web
API with YARP Reverse Proxy) and `EnvelopeGenerator.ReceiverUI`
(Blazor WebAssembly). The API now acts as the single entry point,
proxying requests to the ReceiverUI and external authentication
services.

Redefined the route structure for clarity, introducing sender
routes (`/sender/login`, `/sender`) and receiver routes
(`/envelope/login/{EnvelopeKey}`, `/envelope/{EnvelopeKey}`).
Added multi-envelope support with per-envelope cookies for
simultaneous authentication.

Renamed `EnvelopeViewer` to `EnvelopeReceiver` to reflect its
expanded functionality for viewing and signing envelopes.
Replaced iText7 and PSPDFKit with PDF.js 3.11.174 for document
viewing and signing, with configurable quality settings.

Updated `AuthService` and `SignatureCacheService` to support the
new route structure and multi-envelope authentication. Adjusted
sender login flow to redirect to `/sender` upon success.

Updated documentation to reflect the new architecture, route
structure, and file renames. Deprecated libraries and past
mistakes were documented to avoid repetition.

COPILOT_CONTEXT_EN.md

@@ -7,6 +7,51 @@ Digital document signing system with **unified Blazor WASM frontend** for both S
 
 ---
 
+## Deployment Architecture
+
+**Two Presentation Projects (Both Required):**
+
+1. **EnvelopeGenerator.API** (ASP.NET Core Web API)
+   - Runs independently (development & production)
+   - **YARP Reverse Proxy** configured via `yarp.json`
+   - Proxies requests to:
+     - `EnvelopeGenerator.ReceiverUI` (Blazor WASM)
+     - External Auth.API service
+   - Serves as single entry point for all requests
+
+2. **EnvelopeGenerator.ReceiverUI** (Blazor WebAssembly)
+   - Runs on separate host/port
+   - Accessed **only through API proxy** (not directly)
+   - Serves static files (HTML, JS, CSS, WASM)
+
+**Request Flow:**
+```
+Client ? API:8088 (YARP Proxy) ? ReceiverUI:52936 (Blazor WASM)
+                                ? Auth.API:9090 (External Auth Service)
+```
+
+**Configuration:** `EnvelopeGenerator.API/yarp.json`
+
+---
+
+## ReceiverUI Route Structure
+
+### Sender Routes
+| Route | File | Purpose |
+|---|---|---|
+| `/sender/login` | `LoginSender.razor` | Username/password authentication |
+| `/sender` | `EnvelopeSender.razor` | Sender dashboard (envelope list) |
+
+### Receiver Routes
+| Route | File | Purpose |
+|---|---|---|
+| `/envelope/login/{EnvelopeKey}` | `LoginReceiver.razor` | Access code authentication for specific envelope |
+| `/envelope/{EnvelopeKey}` | `EnvelopeReceiver.razor` | View & sign envelope (PDF.js viewer) |
+
+**Multi-Envelope Support:** Receivers can login to multiple envelopes simultaneously (per-envelope cookie authentication).
+
+---
+
 ## Architecture Evolution
 
 ### Old Architecture (Deprecated)
@@ -42,9 +87,10 @@ Digital document signing system with **unified Blazor WASM frontend** for both S
 
 | File | Route/Purpose |
 |---|---|
-| `ReceiverUI/Pages/EnvelopeViewer.razor` | `/envelope/{key}` — PDF.js viewer (read-only). |
-| `ReceiverUI/Pages/LoginReceiver.razor` | `/login/{EnvelopeKey}` — Access code auth. |
-| `ReceiverUI/Pages/LoginSender.razor` | `/login` — Username/password auth. |
+| `ReceiverUI/Pages/EnvelopeSender.razor` | `/sender` — Sender dashboard (envelope list). |
+| `ReceiverUI/Pages/EnvelopeReceiver.razor` | `/envelope/{key}` — Receiver PDF viewer & signing. |
+| `ReceiverUI/Pages/LoginSender.razor` | `/sender/login` — Sender username/password auth. |
+| `ReceiverUI/Pages/LoginReceiver.razor` | `/envelope/login/{EnvelopeKey}` — Receiver access code auth. |
 | `ReceiverUI/wwwroot/js/pdf-viewer.js` | PDF.js wrapper (zoom, pagination, thumbnails). |
 | `ReceiverUI/wwwroot/js/receiver-signature.js` | Signature pad (draw/type/image). |
 | `ReceiverUI/wwwroot/css/envelope-viewer.css` | EnvelopeViewer styles. |
@@ -83,11 +129,11 @@ Digital document signing system with **unified Blazor WASM frontend** for both S
 
 ---
 
-## EnvelopeViewer — PDF.js Viewer
+## EnvelopeReceiver — PDF.js Viewer & Signing
 
 **Route:** `/envelope/{EnvelopeKey}`  
 **Tech:** PDF.js 3.11.174 + Blazor WASM + configurable quality  
-**File:** `ReceiverUI/Pages/EnvelopeViewer.razor`
+**File:** `ReceiverUI/Pages/EnvelopeReceiver.razor`
 
 ### Key Features
 1. HiDPI/Retina support (4x quality)
@@ -126,7 +172,7 @@ window.pdfViewer = {
 
 ---
 
-## Signature Workflow — EnvelopeViewer
+## Signature Workflow — EnvelopeReceiver
 
 **IMPORTANT:** iText7 NOT used (GPL license issue). Client-side overlay system only.
 
@@ -214,7 +260,7 @@ public class SignatureCacheService {
 
 ## Sender Login
 
-**Route:** `/login`  
+**Route:** `/sender/login`  
 **File:** `ReceiverUI/Pages/LoginSender.razor`  
 **Tech:** Bootstrap 5 + DevExpress Blazing Berry theme
 
@@ -246,7 +292,7 @@ public async Task<SenderLoginResult> LoginSenderAsync(string username, string pa
 ```
 
 **Response:**
-- `200 OK` ? Cookie set, redirect to `/`
+- `200 OK` ? Cookie set, redirect to `/sender`
 - `401 Unauthorized` ? Show error: "Ungültige Anmeldedaten"
 - Other ? Show error: "Serverfehler"
 
@@ -256,16 +302,18 @@ public async Task<SenderLoginResult> LoginSenderAsync(string username, string pa
 1. User enters username + password
 2. Click "Anmelden" or press Enter
 3. Call `AuthService.LoginSenderAsync()`
-4. Success ? `Navigation.NavigateTo("/", forceLoad: true)`
+4. Success ? `Navigation.NavigateTo("/sender", forceLoad: true)`
 5. Error ? Display alert
 
 ---
 
 ## Receiver Login
 
-**Route:** `/login/{EnvelopeKey}`  
+**Route:** `/envelope/login/{EnvelopeKey}`  
 **File:** `ReceiverUI/Pages/LoginReceiver.razor`
 
+**Multi-Envelope Support:** Cookies are stored per-envelope (e.g., `AuthTokenSignFLOWReceiver.{envelopeKey}`), allowing simultaneous authentication for multiple envelopes in the same browser session.
+
 ### AuthService Method
 ```csharp
 public enum EnvelopeLoginResult { Success, InvalidCode, NotFound, Error }
@@ -307,7 +355,7 @@ public async Task<EnvelopeLoginResult> LoginEnvelopeReceiverAsync(string key, st
 
 | Mistake | Why Wrong |
 |---|---|
-| Using iText7 in EnvelopeViewer | GPL license issue. Use overlay system instead. |
+| Using iText7 in EnvelopeReceiver | GPL license issue. Use overlay system instead. |
 | Using PSPDFKit | Removed from architecture. Use PDF.js + DevExpress. |
 | Hardcoded quality values in PDF.js | Use `appsettings.json` for configurability. |
 | Complex toolbar layouts | User wants simplicity. Keep horizontal layout. |
@@ -367,4 +415,4 @@ Proves database uses INCHES natively.
 
 ---
 
-**Last Updated:** Session 17 (Architecture unification documentation)
+**Last Updated:** Session 18 (File renames & routing restructure)