chore: migrate to Rust Edition 2024

- Update edition from 2021 to 2024 in Cargo.toml
- Remove explicit `ref` bindings in pattern matches (di.rs, carddav_adapter.rs)
  Edition 2024 uses implicit ref binding modes
- Refactor folder_handler.rs: change `impl IntoResponse` return types to
  concrete `axum::response::Response` to avoid lifetime capture issues
  (Edition 2024 captures all in-scope lifetimes in `impl Trait`)
- All 101 tests pass, zero warnings

src/common/di.rs

@@ -590,7 +590,7 @@ impl AppServiceFactory {
         };
         
         // 10b. Wire admin settings service when auth + DB are available
-        if let (Some(ref auth_svc), Some(ref pool)) = (&app_state.auth_service, &db_pool) {
+        if let (Some(auth_svc), Some(pool)) = (&app_state.auth_service, &db_pool) {
             let settings_repo = Arc::new(
                 crate::infrastructure::repositories::pg::SettingsPgRepository::new(pool.clone())
             );