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/config.rs

@@ -202,7 +202,7 @@ pub struct UiSettings {
 
 impl UiSettings {
     fn default_theme() -> String {
-        "default".to_string()
+        "default_dark".to_string()
     }
 
     fn default_word_wrap() -> bool {
@@ -388,7 +388,9 @@ mod tests {
         #[cfg(target_os = "macos")]
         {
             // macOS should use ~/Library/Application Support
-            assert!(path.to_string_lossy().contains("Library/Application Support"));
+            assert!(path
+                .to_string_lossy()
+                .contains("Library/Application Support"));
         }
 
         println!("Config conversation path: {}", path.display());