feat: implement utils.rs and state.rs

utils.rs: natural sort, file fingerprinting, library ID computation,
pretty title formatting, path helpers, clamp, truthy (38 tests)

state.rs: atomic JSON persistence with backup rotation and
fallback loading (8 tests)

src-tauri/src/lib.rs

@@ -1,3 +1,6 @@
+pub mod state;
+pub mod utils;
+
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
     tauri::Builder::default()