Prepare for Blazor Server/Auto migration by documenting current
WASM-specific culture initialization approach and providing
detailed migration checklist in COPILOT_CONTEXT.md.
Removed a redundant entry for the root route (`/`) in the
`ReceiverUI Route Structure` section of the documentation.
Both entries referenced the same file (`Index.razor`) and
served the same purpose, making the duplication unnecessary.
Replaced `COPILOT_CONTEXT_EN.md` with `COPILOT_CONTEXT.md`
in the `ProjectSection(SolutionItems)` of the solution file
`EnvelopeGenerator.sln` to reflect updated documentation.
