Whack-a-Mole: proper UI for dark mode + add light mode

### Project slug (e.g., snake, todo)

whack-a-mole

### Summary

The Whack-a-Mole game currently lacks a polished dark theme and has no light mode toggle. Implement a full UI polish for dark mode (typography, contrast, animations) and add a light mode with a theme toggle that persists user choice.

### Details / Acceptance criteria

Dark mode UI looks polished: readable contrast, consistent card/background colors, visible focus states, and smooth mole hit animations.

Light mode implemented and matches overall app design language.

Theme toggle available on the game page and in global header (if present).

Theme choice persists across refreshes (localStorage or equivalent).

Accessibility: meets basic contrast ratio for text/buttons and supports keyboard controls for hitting moles.

Checklist
Design color tokens for dark & light themes.
Implement theme toggle component.
Apply theme tokens to WhackAMole layout (board, moles, HUD, buttons).
Persist theme selection.
Test keyboard controls + contrast.

### Scope

- [x] UI changes
- [ ] Logic changes
- [ ] Accessibility
- [ ] Tests (manual)