Add OGP image for social sharing

[shingokmg]

Overview

Create an OGP image for diff and add the required social meta tags to improve link previews on Facebook, LinkedIn, and X.

Background

• Social sharing previews are not fully prepared yet
• We want the first impression to communicate both ease of use and the fact that input text is not sent to the server

Tasks

• Create a 1200x630 OGP image
• Add OGP meta tags to the top page
• Add Twitter Card meta tags for X
• Verify the preview on major social platforms

Key messages

• Japanese text diff comparison tool
• Quick and easy to use
• Useful for checking changes made by AI or other people
• Input text is not sent to the server

Notes

• Keep the visual tone bright, trustworthy, and close to GitHub light mode
• The image should feel like a practical tool for Japanese text, not a code diff tool