remove MIGRATION_CONTEXT.md

Migrate ReceiverUI to hybrid Blazor WebUI architecture

Migrated the `EnvelopeGenerator.ReceiverUI` project to a new
hybrid Blazor architecture (`EnvelopeGenerator.WebUI`) that
supports both Blazor Server and WebAssembly (WASM) modes.

- Added `WebUI` (server) and `WebUI.Client` (WASM) projects.
- Migrated client-side pages to `WebUI.Client` with
  `@rendermode InteractiveWebAssembly`.
- Migrated server-side pages to `WebUI` with
  `@rendermode InteractiveServer`.
- Added YARP reverse proxy (`yarp.json`) to `WebUI` for API
  and Swagger routing.
- Registered DevExpress server-side services in `WebUI` to
  enable backend rendering for `DxPdfViewer`.
- Migrated services, models, options, and data files to
  `WebUI.Client` with updated namespaces.
- Merged static files (JS, CSS, configuration) from
  `ReceiverUI/wwwroot` to `WebUI/wwwroot`.
- Retained `ReceiverUI` project for rollback safety.

This migration resolves the issue where the DevExpress
`DxPdfViewer` failed to render PDFs in a pure Blazor
WebAssembly environment due to missing server-side rendering
services.

EnvelopeGenerator.sln

@@ -23,7 +23,6 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{134D4164-B29
 	ProjectSection(SolutionItems) = preProject
 		COPILOT_CONTEXT.md = COPILOT_CONTEXT.md
 		FORM_APPLICATION_CONTEXT.md = FORM_APPLICATION_CONTEXT.md
-		MIGRATION_CONTEXT.md = MIGRATION_CONTEXT.md
 	EndProjectSection
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "tests", "tests", "{0CBC2432-A561-4440-89BC-671B66A24146}"