Add persisted dark theme support

[Jorel97]

fixes #1

Summary

• add an early theme bootstrap in the document head so persisted/system dark mode is applied before the stylesheet loads
• add a header theme toggle that persists light/dark in localStorage under taskforge.theme
• add dark-mode CSS custom properties across the shell, form, task rows, messages, and controls
• add focused tests for theme preference resolution and root application

Validation

• npm test -> 7 tests passed
• npm run lint -> passed
• node --check src/app.js
• node --check src/theme.mjs
• node --check test/theme.test.js
• git diff --check

[codebounty-dev-bot]

⚠️ Attention Sponsor: This PR was submitted by @Jorel97 who has not applied for this bounty (#1). Please verify if you want to consider this contribution.

👋 Hey @Jorel97: This issue has a bounty on CodeBounty! To be eligible for the reward, please sign up on CodeBounty and apply for this bounty. If you apply, your PR will be automatically associated with your application.

[codebounty-dev-platform]

📝 Thank you for applying to this bounty! Your PR has been associated with your application. The sponsor will review your application soon.