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/gruvbox.toml

@@ -0,0 +1,23 @@
+name = "gruvbox"
+text = "#ebdbb2"
+background = "#282828"
+focused_panel_border = "#fe8019"
+unfocused_panel_border = "#7c6f64"
+user_message_role = "#b8bb26"
+assistant_message_role = "#83a598"
+thinking_panel_title = "#d3869b"
+command_bar_background = "#3c3836"
+status_background = "#3c3836"
+mode_normal = "#83a598"
+mode_editing = "#b8bb26"
+mode_model_selection = "#fabd2f"
+mode_provider_selection = "#8ec07c"
+mode_help = "#d3869b"
+mode_visual = "#fe8019"
+mode_command = "#fabd2f"
+selection_bg = "#504945"
+selection_fg = "#ebdbb2"
+cursor = "#fe8019"
+placeholder = "#665c54"
+error = "#fb4934"
+info = "#b8bb26"