refactor(tauri): decompose lib command surface into domain modules

Split the large lib command module into nested app_api, cache, sync, and ui submodules while preserving command signatures and behavior. This keeps command responsibilities isolated and makes further extraction safer.

src-tauri/src/lib_commands/cache/mod.rs

@@ -0,0 +1,9 @@
+use super::*;
+
+mod offline;
+mod downloads;
+mod hot;
+
+pub(crate) use offline::*;
+pub(crate) use downloads::*;
+pub(crate) use hot::*;