Refactor architecture and optimize PDF rendering

Refactor `EnvelopeSenderPage.razor` to focus on UI composition
and state orchestration, avoiding heavy domain logic. Adopt a
modular page architecture to improve maintainability by splitting
large pages into smaller subcomponents and centralizing shared
logic. Conduct a performance review of sender/receiver flows to
optimize route transitions, component lifecycle behavior, and
loading-state correctness.

Audit `Services` for redundant request/payload remapping and
eliminate unnecessary transformations. Centralize dependency
injection registrations into dedicated extension methods to
reduce startup composition sprawl.

Refactor `PdfRenderController.cs` and its client service contract
to replace JSON/base64 transport for PDF rendering with binary
streaming (`application/octet-stream` or multipart). Add a `TODO`
comment to highlight the need for further optimization of large
PDF handling to reduce payload size and memory pressure.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Controllers/PdfRenderController.cs

@@ -11,6 +11,7 @@ namespace EnvelopeGenerator.Server.Controllers;
 [Route("api/[controller]")]
 public class PdfRenderController(IPdfPageImageRenderer renderer) : ControllerBase
 {
+    // TODO(perf): Replace JSON/base64 transport for PdfBytes with application/octet-stream (or multipart) and stream-based request parsing to reduce payload size and allocation pressure for large PDFs.
     [HttpPost("pages")]
     [Authorize(AuthenticationSchemes = AuthScheme.Sender)]
     public async Task<IActionResult> RenderPages([FromBody] PdfRenderRequestDto? request, CancellationToken cancel)