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/Cargo.toml

@@ -24,6 +24,7 @@ async-trait = "0.1.73"
 toml = "0.8.0"
 shellexpand = "3.1.0"
 dirs = "5.0"
+ratatui = { workspace = true }
 
 [dev-dependencies]
 tokio-test = { workspace = true }