Add persisted dark theme support

[newmattock]

Fixes #1.

Summary

• add a header theme toggle that switches between light and dark mode
• resolve the initial theme from taskforge.theme or prefers-color-scheme before the stylesheet loads to avoid dark-mode flash
• persist manual choices in localStorage under taskforge.theme
• move theme behavior into a focused module with Node test coverage
• add an ESLint flat config so the lint script runs on the current ESLint version

Verification

• node --test
• node --check src/theme.js src/app.js test/theme.test.js eslint.config.mjs
• eslint src test
• git diff --check
• live server smoke: confirmed the served HTML includes the pre-stylesheet theme resolver and toggle markup, src/theme.js exposes taskforge.theme/system-preference handling, and styles.css serves dark-theme variables plus responsive toggle styling

Notes

• Playwright package was available only without browser binaries in this environment, so the browser smoke used the local HTTP server plus static served-asset checks and unit coverage for theme resolution/toggle persistence.

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[newmattock]

Resolved the merge conflicts from the merged OAuth work and pushed the updated branch. Validation on the rebased branch: npm test (12 passed), npm run lint, and git diff --cached --check. The PR is mergeable again.

[newmattock]

Following up on the dark theme bounty submission. PR #21 is still mergeable and associated with my CodeBounty application; validation remains npm test, npm run lint, and git diff --check from the rebased branch. Please let me know if you want any adjustments while it is under review.