refactor: restructure application to CQRS, update api-types + presentation

- application: replace flat use_cases/ with identity/{commands,queries}/ and organization/commands/
- each use case now split into Command/Query struct + Handler struct
- api-types: add username to RegisterRequest/UserResponse, add CreateAlbumRequest/AlbumResponse
- presentation: update state, handlers, factory to use new handler types
- tests: restructured to match CQRS module layout, added get_profile tests

crates/application/tests/organization/commands/create_album.rs

@@ -0,0 +1,30 @@
+use std::sync::Arc;
+use application::testing::InMemoryAlbumRepository;
+use application::organization::{CreateAlbumCommand, CreateAlbumHandler};
+use domain::errors::DomainError;
+use domain::value_objects::SystemId;
+
+#[tokio::test]
+async fn creates_album() {
+    let repo = Arc::new(InMemoryAlbumRepository::new());
+    let handler = CreateAlbumHandler::new(repo);
+    let creator = SystemId::new();
+    let album = handler.execute(CreateAlbumCommand {
+        title: "Vacation 2024".into(),
+        creator_id: creator,
+    }).await.unwrap();
+    assert_eq!(album.title, "Vacation 2024");
+    assert_eq!(album.creator_user_id, creator);
+    assert_eq!(album.asset_count(), 0);
+}
+
+#[tokio::test]
+async fn rejects_empty_title() {
+    let repo = Arc::new(InMemoryAlbumRepository::new());
+    let handler = CreateAlbumHandler::new(repo);
+    let result = handler.execute(CreateAlbumCommand {
+        title: "".into(),
+        creator_id: SystemId::new(),
+    }).await;
+    assert!(matches!(result, Err(DomainError::Validation(_))));
+}