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/gemini/prompt.rs

@@ -29,12 +29,13 @@ impl PromptBuilder {
         }
     }
 
-    pub fn build_categorization_prompt(&self) -> String {
+    pub fn build_categorization_prompt(&self, categories: &[String]) -> String {
+        let categories_str = categories.join("', '");
         format!(
             "I have these files in my Downloads folder: [{}]. \
-             Categorize them into these folders: 'Images', 'Documents', 'Installers', 'Music', 'Archives', 'Code', 'Misc'. \
+             Categorize them into these folders: '{}'. \
              Return ONLY a JSON object with this structure: {{ 'files': [ {{ 'filename': 'name', 'category': 'folder' }} ] }}",
-            self.file_list
+            self.file_list, categories_str
         )
     }