Add persisted theme toggle helper

[tristanwork]

Summary

• add a framework-agnostic theme toggle helper for navbar mounting
• persist light/dark preference in localStorage
• apply theme state to document attributes/classes and button accessibility labels
• add no-dependency tests for storage, initial theme, DOM application, and click toggling

Verification

• npm test

Field	Value
Bounty	$50 USD1
Wallet	0x742d35Cc6634C0532925a3b844Bc9e7595f0bEb7

Fixes #19

[tristanwork]

Follow-up after commit b4ad3fc:

• Added installNavbarThemeToggle() so the toggle can be installed directly into a navbar.
• Added smooth transition CSS injection for background, text, and border color changes.
• Added theme-demo.html, which shows the visible navbar toggle in a browser page.
• Expanded tests for style injection, idempotent install, and navbar mounting.

Local verification: npm test passes with parser tests 6/6 and theme toggle tests 27/27.