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/solarized.toml

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+name = "solarized"
+text = "#839496"
+background = "#002b36"
+focused_panel_border = "#268bd2"
+unfocused_panel_border = "#073642"
+user_message_role = "#2aa198"
+assistant_message_role = "#cb4b16"
+thinking_panel_title = "#6c71c4"
+command_bar_background = "#073642"
+status_background = "#073642"
+mode_normal = "#268bd2"
+mode_editing = "#859900"
+mode_model_selection = "#b58900"
+mode_provider_selection = "#2aa198"
+mode_help = "#6c71c4"
+mode_visual = "#d33682"
+mode_command = "#b58900"
+selection_bg = "#073642"
+selection_fg = "#93a1a1"
+cursor = "#d33682"
+placeholder = "#586e75"
+error = "#dc322f"
+info = "#859900"