fix(blog): sanitize post HTML to prevent stored XSS (fixes #57)

[FuturMix]

Adds sanitize-html to sanitize blog post HTML before rendering via dangerouslySetInnerHTML.

The post.html field from the autoblog webhook is currently rendered unsanitized. If the upstream source is compromised, arbitrary JS can execute in visitor browsers.

Changes:

• Add sanitize-html dependency and @types/sanitize-html
• Sanitize post.html in the blog post component with allowed tags (img, h1-h3) and safe attributes

Fixes #57

[FuturMix]

@ralyodio This is a critical stored XSS vulnerability — blog post HTML from the autoblog webhook is rendered unsanitized. Could you review and merge this fix? We also have 6 other security fixes ready (PR #60, #62, #66, #67, #68, #70).