Add custom categories feature

- Add support for user-defined custom categories in config.toml
- Update Config struct with categories field and default_categories() function
- Thread categories through GeminiClient and prompt builder
- Update AI prompts to use dynamic categories instead of hardcoded ones
- Add comprehensive documentation with examples for different use cases
- Update tests to support new categories field
- Maintain backward compatibility with default categories
- Update version from 1.0.3 to 1.0.4

Closes feature request for custom categories.

src/settings/mod.rs

@@ -1,7 +1,9 @@
 pub mod config;
 pub mod prompt;
 
-pub use config::{Config, get_or_prompt_api_key, get_or_prompt_download_folder};
+pub use config::{
+    Config, get_or_prompt_api_key, get_or_prompt_config, get_or_prompt_download_folder,
+};
 pub use prompt::Prompter;
 
 #[cfg(test)]