🔒 Fix XSS vulnerability by replacing @html with a secure dompurify fragment action

[Sparkier]

🎯 What: The vulnerability fixed
The raw @html content tag was being used to render blog content, which poses a Cross-Site Scripting (XSS) vulnerability, even when the HTML is pre-sanitized due to DOM mutation-based bypasses.

⚠️ Risk: The potential impact if left unfixed
If an attacker can inject malicious payloads into the rendered Markdown (e.g. through a dompurify bypass or stored payload), the code will be executed in the user's browser, allowing unauthorized access, session hijacking, or defacement.

🛡️ Solution: How the fix addresses the vulnerability
Replaced the {@html} rendering with a custom Svelte action, renderSanitizedHtml. This action parses the HTML safely utilizing isomorphic-dompurify configured to return a direct DocumentFragment ({ RETURN_DOM_FRAGMENT: true }), preventing the string-to-DOM conversion typically exploited in mutation XSS. The fragment is then securely injected into the container via replaceChildren(). Tests were added to ensure this action accurately purifies inputs while maintaining content updating dynamically.

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PR created automatically by Jules for task 8729800088616993268 started by @Sparkier

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