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

Cargo.toml

@@ -8,5 +8,13 @@ serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 clap = { version = "4.5", features = ["derive"] }
 crossterm = "0.28"
-
 Inflector = { version = "0.11", default-features = false }
+anyhow = "1.0"
+log = "0.4"
+env_logger = "0.11"
+indicatif = "0.17"
+which = "6.0"
+[dev-dependencies]
+assert_cmd = "2.0"
+tempfile = "3.10"
+predicates = "3.1"