Unify Blazor frontend and API into EnvelopeGenerator.Server

The `EnvelopeGenerator.Server` project now serves as the unified runtime host for both the Blazor frontend and Web API backend, deprecating the `EnvelopeGenerator.API` project. All API endpoints have been merged into `EnvelopeGenerator.Server`.

Key changes:
- Updated `EnvelopeGenerator.Server` to include API controllers.
- Deprecated `EnvelopeGenerator.API` and removed it from the active solution.
- Simplified development workflow to run only `EnvelopeGenerator.Server`.
- Updated `appsettings.json` to consolidate configuration settings.
- Adjusted documentation to reflect the new unified architecture.
- Updated manual testing steps and environment variable references.
- Migrated legacy VB.NET app references to the new architecture.

These changes simplify development, deployment, and testing workflows while consolidating the solution architecture.

FORM_APPLICATION_CONTEXT.md

@@ -1,7 +1,7 @@
 # EnvelopeGenerator.Form — VB.NET Desktop Application Context
 
 ## Purpose
-**Legacy Windows Forms application** for envelope creation, management, and signature field placement. Built with **DevExpress components** and **GdPicture14** for PDF manipulation. This application is being **migrated to EnvelopeGenerator.Server + API** architecture.
+**Legacy Windows Forms application** for envelope creation, management, and signature field placement. Built with **DevExpress components** and **GdPicture14** for PDF manipulation. This application is being **migrated to `EnvelopeGenerator.Server` architecture** (merged Blazor + Web API host).
 
 **Primary Libraries:** DevExpress XtraGrid/XtraEditors, GdPicture14, VB.NET (.NET Framework 4.6.2)
 
@@ -368,7 +368,7 @@ End Sub
 
 ---
 
-## Migration to EnvelopeGenerator.Server + API
+## Migration to EnvelopeGenerator.Server
 
 ### Mapping Table