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/audible-util.iml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<module type="EMPTY_MODULE" version="4">
+  <component name="NewModuleRootManager">
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
+      <excludeFolder url="file://$MODULE_DIR$/target" />
+    </content>
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+</module>