Add persistent dark theme support

[victorhblancom-design]

Fixes #1.

Summary

• Applies the initial theme before the stylesheet loads, using saved taskforge.theme when present and prefers-color-scheme otherwise.
• Adds a visible header theme toggle that persists explicit light/dark choices in localStorage.
• Moves the light/dark palette into CSS custom properties and themes form, status, item, and danger states without adding a framework.

Verification

• node --test
• node --check src/app.js
• Headless Edge smoke: dark system preference on first load, toggle persistence after reload, light system preference on first load, and pre-CSS theme bootstrap order.

[codebounty-dev-bot]

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[victorhblancom-design]

This is my implementation for the #1 bounty. CodeBounty could not automatically associate the application, so please consider this PR for manual sponsor review/association if eligible.