Read picked import files through the fs plugin so Android content URIs work

On Android the open dialog returns a content:// URI, which std::fs can't
open, so ManaBox/library and deck imports failed with "no such file or
directory". Read them via tauri-plugin-fs, which resolves content URIs and
falls through to plain paths on desktop.

Closes #24

src-tauri/src/lib.rs

@@ -37,6 +37,7 @@ pub(crate) fn logged<T>(what: &str, result: Result<T, String>) -> Result<T, Stri
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
     let builder = tauri::Builder::default().plugin(tauri_plugin_dialog::init())
+        .plugin(tauri_plugin_fs::init())
         .plugin(tauri_plugin_opener::init());
     // Android gets sync sign-ins back through the `mtgc://` scheme; desktop
     // uses a loopback port instead and needs no scheme registered.