## Why

The current UI is locked to dark presentation, which does not match all user preferences or accessibility needs. Adding multi-theme support now improves usability and lets returning users keep a consistent visual experience.

## What Changes

- Add a theme switcher in the top-right header area.
- Support four theme modes: **system**, **light**, **dark**, and **high-contrast**.
- Render theme options as icons (not text-only controls).
- Persist selected theme in client storage with **localStorage as primary** and **cookie fallback**.
- Restore persisted theme for returning users.
- Use **system** as default when no prior selection exists.

## Capabilities

### New Capabilities
- `theme-switcher-control`: Provide an icon-based theme switcher in the header with system/light/dark/high-contrast options.
- `theme-preference-persistence`: Persist and restore user-selected theme using localStorage first, with cookie fallback.
- `theme-default-system`: Apply system theme automatically when no saved preference exists.

### Modified Capabilities
- None.

## Impact

- **Frontend/UI:** Header controls and global styling system updated for four theme modes.
- **State management:** Client-side preference state handling added for theme selection and restoration.
- **Accessibility:** High-contrast option improves readability for users needing stronger contrast.
- **APIs/Backend:** No required backend API changes expected.
