Add bookmarks sidebar and MV3 CSP cleanup

[Niansi]

Summary

• Add an optional bookmarks bar sidebar to the Tab Out new tab dashboard
• Hide the bookmarks sidebar when the bookmarks bar has no links
• Replace inline event handlers with CSP-safe event listeners for Manifest V3
• Update README attribution to preserve Zara as the original author and note this optimized fork

Testing

• node --check extension/app.js
• node --check extension/background.js
• Parsed extension/manifest.json with Node JSON.parse

Original project by Zara. Optimizations by wuxiangyu05 / Niansi.

[Niansi]

Thanks for taking a look. I noticed #59 covers a similar bookmarks + CSP-safe direction. This PR is intended as a smaller, more focused alternative: it keeps the existing Tab Out layout mostly intact, adds only an optional left bookmarks bar sidebar, hides that sidebar when the bookmarks bar has no links, and removes the MV3 CSP violations from inline event handlers.

Original authorship remains with Zara; this is just a lightweight optimization/fork contribution.