Add persistent dark theme support

[TN0721]

Closes #1

Adds persisted dark theme support for TaskForge.

Changes:

• Adds a header theme toggle for switching light/dark mode
• Respects system prefers-color-scheme on first load
• Persists the user's selected theme in localStorage under taskforge.theme
• Uses CSS custom properties for light and dark palettes
• Updates the README now that dark theme support is available

Verification:

• node --check src/app.js
• npx prettier --check index.html styles.css src/app.js README.md
• git diff --check

Note: npm run lint currently fails because the project does not include an ESLint 9 config file.

[codebounty-dev-bot]

⚠️ Attention Sponsor: This PR was submitted by @TN0721 who has applied for bounty #1 but has not been accepted yet. Please review their application and accept it if you want to consider this PR.

👋 Hey @TN0721: Your application for this bounty on CodeBounty is pending review. The sponsor will need to accept your application for you to be eligible for the reward.