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_light.toml

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+name = "default_light"
+text = "black"
+background = "white"
+focused_panel_border = "#4a90e2"
+unfocused_panel_border = "#dddddd"
+user_message_role = "#0055a4"
+assistant_message_role = "#8e44ad"
+thinking_panel_title = "#8e44ad"
+command_bar_background = "white"
+status_background = "white"
+mode_normal = "#0055a4"
+mode_editing = "#2e8b57"
+mode_model_selection = "#b58900"
+mode_provider_selection = "#008b8b"
+mode_help = "#8e44ad"
+mode_visual = "#8e44ad"
+mode_command = "#b58900"
+selection_bg = "#a4c8f0"
+selection_fg = "black"
+cursor = "#d95f02"
+placeholder = "gray"
+error = "#c0392b"
+info = "green"