feat: improve output-path handling and error messages

- If --output-path is a directory, output file uses default naming in that directory
- If --output-path is a file, use as-is
- Improved directory existence/writability checks and error messages
- Updated CLI help and documentation to describe new behavior
- Cleaned up unused imports and resolved all warnings
- Ensured ffmpeg failures are clearly reported to the user

.idea/modules.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectModuleManager">
+    <modules>
+      <module fileurl="file://$PROJECT_DIR$/.idea/audible-util.iml" filepath="$PROJECT_DIR$/.idea/audible-util.iml" />
+    </modules>
+  </component>
+</project>