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

Cargo.lock

@@ -2448,6 +2448,7 @@ dependencies = [
  "tauri-build",
  "tauri-plugin-deep-link",
  "tauri-plugin-dialog",
+ "tauri-plugin-fs",
  "tauri-plugin-opener",
  "url",
 ]