feat: JSON import + mapping presets in SPA

- Accept .json files in import upload, send format to backend
- Backend endpoint PUT /import/sessions/{id}/profile/{profile_id}
- Load saved presets on mapping step, auto-apply and skip to preview
- Save current mapping as preset on confirm step
- Delete presets from mapping step

crates/presentation/src/routes.rs

@@ -365,6 +365,10 @@ fn api_routes(rate_limit: u64) -> Router<AppState> {
             "/import/profiles/{id}",
             routing::delete(handlers::import::api_delete_profile),
         )
+        .route(
+            "/import/sessions/{id}/profile/{profile_id}",
+            routing::put(handlers::import::api_apply_profile),
+        )
         .route(
             "/profile",
             routing::get(handlers::users::get_profile).put(handlers::users::update_profile_handler),