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.

AGENTS.md

@@ -245,6 +245,7 @@ Manual testing workflow:
 - Perform a senior-level page performance review across sender/receiver flows before further UX changes: verify route transitions, component mount/unmount behavior, loading-state correctness, duplicate API calls, and perceived latency (first meaningful paint and return-navigation responsiveness). Treat loading logic defects as architecture/performance issues first, not styling problems.
 - Audit `EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/` for unnecessary request/payload remapping and remove redundant mappings where direct request DTO usage is safe; review client HTTP methods end-to-end for similar avoidable transformation layers.
 - Centralize dependency injection registrations into dedicated extension methods/modules (especially server + client service registrations in `Program.cs`) to reduce startup composition sprawl and improve maintainability.
+- Refactor `EnvelopeGenerator.Server/EnvelopeGenerator.Server/Controllers/PdfRenderController.cs` + matching client service contract to remove JSON/base64 `byte[]` transport for PDF rendering; prefer binary streaming (`application/octet-stream` or multipart), avoid `data:` URL bloat where possible, and validate end-to-end memory/latency behavior with large PDFs.
 
 ## Database
 

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)