Add theme support for TUI

[krjordan]

Implement theming system for customizable TUI appearance.

Requirements

• Define theme structure (colors, borders, symbols)
• Ship with default light/dark themes
• Load custom themes from config
• Theme hot-reload without restart
• Accessible color choices

Acceptance Criteria

• Default dark and light themes included
• Themes defined in TOML config
• Can switch themes with shortcut key
• Custom themes in ~/.config/pulseprint/themes/
• High contrast theme for accessibility
• Theme preview in settings