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.

themes/monokai.toml

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+name = "monokai"
+text = "#f8f8f2"
+background = "#272822"
+focused_panel_border = "#f92672"
+unfocused_panel_border = "#75715e"
+user_message_role = "#66d9ef"
+assistant_message_role = "#ae81ff"
+thinking_panel_title = "#e6db74"
+command_bar_background = "#272822"
+status_background = "#272822"
+mode_normal = "#66d9ef"
+mode_editing = "#a6e22e"
+mode_model_selection = "#e6db74"
+mode_provider_selection = "#66d9ef"
+mode_help = "#ae81ff"
+mode_visual = "#f92672"
+mode_command = "#e6db74"
+selection_bg = "#75715e"
+selection_fg = "#f8f8f2"
+cursor = "#f92672"
+placeholder = "#75715e"
+error = "#f92672"
+info = "#a6e22e"