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.

COPILOT_CONTEXT.md

@@ -39,7 +39,7 @@ This project contains:
 - `EnvelopeGenerator.Domain` — domain models, constants, shared abstractions
 - `EnvelopeGenerator.Infrastructure` — EF Core and infrastructure services
 - `EnvelopeGenerator.PdfEditor` — PDF-related backend utilities
-- `EnvelopeGenerator.API` — still exists in the solution, but the current merged app host is `EnvelopeGenerator.Server`
+- `EnvelopeGenerator.API` — deprecated project (no longer part of the active solution/runtime path)
 
 ### Legacy / Do Not Touch
 - `EnvelopeGenerator.Service`
@@ -121,7 +121,7 @@ Current controller set includes:
 - `SignatureController`
 - `TfaRegistrationController`
 
-Do not assume API behavior lives only in `EnvelopeGenerator.API`; the active merged host contains controller endpoints directly.
+Do not assume API behavior lives in `EnvelopeGenerator.API`; the active merged host contains controller endpoints directly.
 
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