Add built-in theme support with various pre-defined themes

- Introduce multiple built-in themes (`default_dark`, `default_light`, `gruvbox`, `dracula`, `solarized`, `midnight-ocean`, `rose-pine`, `monokai`, `material-dark`, `material-light`).
- Implement theming system with customizable color schemes for all UI components in the TUI.
- Include documentation for themes in `themes/README.md`.
- Add fallback mechanisms for default themes in case of parsing errors.
- Support custom themes with overrides via configuration.

crates/owlen-core/src/conversation.rs

@@ -277,20 +277,26 @@ impl ConversationManager {
         &self,
         storage: &StorageManager,
         name: Option<String>,
-        description: Option<String>
+        description: Option<String>,
     ) -> Result<PathBuf> {
         storage.save_conversation_with_description(&self.active, name, description)
     }
 
     /// Load a conversation from disk and make it active
-    pub fn load_from_disk(&mut self, storage: &StorageManager, path: impl AsRef<Path>) -> Result<()> {
+    pub fn load_from_disk(
+        &mut self,
+        storage: &StorageManager,
+        path: impl AsRef<Path>,
+    ) -> Result<()> {
         let conversation = storage.load_conversation(path)?;
         self.load(conversation);
         Ok(())
     }
 
     /// List all saved sessions
-    pub fn list_saved_sessions(storage: &StorageManager) -> Result<Vec<crate::storage::SessionMeta>> {
+    pub fn list_saved_sessions(
+        storage: &StorageManager,
+    ) -> Result<Vec<crate::storage::SessionMeta>> {
         storage.list_sessions()
     }
 }