refactor: extract view-model mappers from presentation handlers

Move mapping logic (domain→DTO/template structs) into mappers/ module.
Handlers now call mapper fns instead of inline conversions.

crates/presentation/src/lib.rs

@@ -4,6 +4,7 @@ pub mod extractors;
 pub mod factory;
 pub mod forms;
 pub mod handlers;
+pub mod mappers;
 pub mod openapi;
 pub mod ports;
 pub mod render;