Add persisted dark theme support

[wujinhh]

Summary

• add a header theme toggle for light/dark mode
• respect system color-scheme on first load and persist manual choice in localStorage under taskforge.theme
• switch colors to CSS custom properties and add a minimal ESLint 9 config so the existing lint script runs

Verification

• npm run lint
• npx prettier --check index.html styles.css src/app.js README.md eslint.config.mjs
• git diff --check
• manually verified theme toggle and reload persistence in browser

fixes #1

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