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/mappers/mod.rs

@@ -0,0 +1,5 @@
+pub mod diary;
+pub mod import;
+pub mod integrations;
+pub mod movies;
+pub mod users;