Add CodeDiff Checker, CSS Grid Generator, and CSS Flex Generator

[masakazuimai]

What's added

Three free, browser-based developer tools:

Diffing

• CodeDiff Checker — Highlight differences in code or text with character-level precision. Unique feature: detects full-width space contamination (common in Japanese/CJK development environments).

Generators

• CSS Grid Generator — Visual CSS Grid layout generator. Drag to place grid items and auto-generate HTML/CSS code.
• CSS Flex Generator — Visual CSS Flexbox layout generator. Configure flex properties with real-time preview and auto-generate CSS code.

All three tools are:

• ✅ Free to use
• ✅ No signup required
• ✅ Run entirely in the browser (no data sent to servers)
• ✅ One-click code copy

Entries are placed in alphabetical order within their respective sections.

[github-actions]

Hey masakazuimai! Thanks for your submission! 🚀

To keep awesome-devtools instantly accessible for everyone, all tools must be:

• Browser-based — no downloads or native app installs
• Direct link — goes straight to the tool (not a multi-tool landing page)
• 100% free — no signups, trials, paywalls, or usage limits

Please make sure your PR meets these criteria. It may take some time for your submission to be reviewed — you can occasionally comment with a "bump" to bring more attention to it.

Thanks for contributing! 🙌