Add dark theme support

[nguyenduc071912]

Summary

Adds dark theme support using CSS custom properties and a persisted user override.

• Applies the initial theme before loading CSS to avoid a dark-theme flash
• Respects prefers-color-scheme: dark when no explicit choice exists
• Adds a header toggle to switch light/dark modes
• Persists the override in localStorage under taskforge.theme
• Converts remaining hard-coded light colors to theme variables

fixes #1

Tests

npm test
npm run lint

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