refactor: use Cow<str> for errors

since some errors are just string literals;
this lets us avoid allocating a new string

src/utils.rs

@@ -34,7 +34,7 @@ fn load_image_impl(path: impl AsRef<Path>) -> ImageResult<RgbaImage> {
 
     Ok(decoded_image.into_rgba8())
 }
-pub async fn pick_image() -> Result<PathBuf, String> {
+pub async fn pick_image() -> Result<PathBuf, Cow<'static, str>> {
     let Some(filehandle) = AsyncFileDialog::new()
         .add_filter(
             "image",
@@ -52,7 +52,7 @@ pub async fn pick_image() -> Result<PathBuf, String> {
     Ok(filehandle.path().to_owned())
 }
 
-pub async fn pick_save_path() -> Result<PathBuf, String> {
+pub async fn pick_save_path() -> Result<PathBuf, Cow<'static, str>> {
     let Some(filehandle) = AsyncFileDialog::new().save_file().await else {
         return Err("no path to save provided".into());
     };