feat(error): add Rust AppError enum and migrate command modules

- AppError enum with 10 variants (Database, Auth, Filesystem, Ipc, NotFound,
  Validation, Sidecar, Config, Network, Internal) + serde tag serialization
- From impls for rusqlite::Error, std::io::Error, serde_json::Error
- Migrated 9 command modules from Result<T, String> to Result<T, AppError>
- Frontend receives structured {kind, detail} objects via IPC

src-tauri/src/lib.rs

@@ -1,6 +1,7 @@
 mod btmsg;
 mod bttask;
 mod commands;
+pub mod error;
 mod ctx;
 mod event_sink;
 mod fs_watcher;