Add directory selection for saving converted files

Introduce logic and UI for selecting and persisting a directory to save converted files. Extend existing components with directory selection buttons, message updates, and storage preferences.

internal/utils/path.go

@@ -0,0 +1,19 @@
+package utils
+
+import (
+	"fyne.io/fyne/v2"
+	"fyne.io/fyne/v2/storage"
+)
+
+func PathToListableURI(path string) (fyne.ListableURI, error) {
+	if len(path) > 0 {
+		path = "file://" + path
+	}
+
+	uri, err := storage.ParseURI(path)
+	if err != nil {
+		return nil, err
+	}
+
+	return storage.ListerForURI(uri)
+}