#18831: feat(tui): add light theme support for light terminal backgrounds

## Summary Adds `--theme` flag to `openclaw tui` with three modes: - `dark` (default): existing dark theme - `light`: new light-friendly palette - `auto`: detect terminal background via OSC 11 ## Changes - `src/tui/theme/theme.ts`: Add light theme colors and auto-detection - `src/cli/tui-cli.ts`: Add `--theme` CLI flag - `src/tui/theme/syntax-theme.ts`: Export theme mode type ## Testing Tested with GNOME Terminal (Tango Light theme): - `openclaw tui --theme light` - readable on light backgrounds - `openclaw tui --theme auto` - correctly detects and applies theme Closes #16067