Workstream C: convert commands/download to a folder module, extract pinning and smart-cache commands

- Move commands/download.rs to commands/download/mod.rs.
- Extract pin/unpin/is_pinned into download/pinning.rs.
- Extract smart-cache stats/config + album recommendation commands into
  download/smart_cache.rs.
- Re-exported via pub use so command names stay at commands::download::*;
  invoke_handler unchanged, all tests pass.

src-tauri/src/lib.rs

@@ -12,7 +12,7 @@ mod session_poller;
 pub mod settings;
 mod storage;
 mod thumbnail;
-mod utils;
+pub mod utils;
 
 use std::sync::{Arc, Mutex};
 use tokio::sync::Mutex as TokioMutex;