Add Color Themes Configuration

Add support for multiple color themes (light, dark, dracula, monokai) that users can select via command-line flag or config file.

Tasks:

• Create a themes.go file in internal/ui/ with theme definitions
• Add theme constants for popular themes (dracula, monokai, nord, gruvbox)
• Add --theme flag to CLI
• Update config.yaml to include theme option
• Apply theme colors to all lipgloss styles

Acceptance Criteria:

• User can run devstandup --theme dracula
• Theme persists when set in config file
• All UI elements respect the selected theme

Resources:

• Lipgloss Color Documentation: https://github.com/charmbracelet/lipgloss#colors
• Example themes: https://github.com/mbadolato/iTerm2-Color-Schemes