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/default_dark.toml

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+name = "default_dark"
+text = "white"
+background = "black"
+focused_panel_border = "lightmagenta"
+unfocused_panel_border = "#5f1487"
+user_message_role = "lightblue"
+assistant_message_role = "yellow"
+thinking_panel_title = "lightmagenta"
+command_bar_background = "black"
+status_background = "black"
+mode_normal = "lightblue"
+mode_editing = "lightgreen"
+mode_model_selection = "lightyellow"
+mode_provider_selection = "lightcyan"
+mode_help = "lightmagenta"
+mode_visual = "magenta"
+mode_command = "yellow"
+selection_bg = "lightblue"
+selection_fg = "black"
+cursor = "magenta"
+placeholder = "darkgray"
+error = "red"
+info = "lightgreen"