Add persistent dark theme toggle

[VexCode24]

Fixes #1.

Summary:

• Adds a pre-CSS theme bootstrap so dark mode is applied before first paint.
• Adds a header theme toggle that persists explicit user choice in localStorage under taskforge.theme.
• Moves the palette to CSS custom properties and updates the README feature list.

Verification:

• git diff --check
• Browser check at http://127.0.0.1:8123/: toggle switches light/dark, aria state updates, theme persists after reload, task creation still works, and no console errors were reported.

[codebounty-dev-bot]

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[VexCode24]

Adding the exact bounty reference syntax requested on the issue: fixes #1