MIND-13: Basic theming + accessibility pass

Summary

Apply a dark theme by default and ensure the UI is accessible by basic standards.

Tasks

• Configure ThemeData for dark theme defaults.
• Ensure color contrast meets WCAG AA requirements.
• Verify touch targets are ≥ 48dp.
• Add semantic labels for screen readers.
• Ensure font scaling up to 200% is supported.

Acceptance Criteria

• App runs in dark theme by default.
• UI elements meet accessibility standards.
• Timer and controls are accessible via screen readers.
• Layout remains usable with large font sizes.