🛡️ Sentinel: [CRITICAL] Fix DOMPurify bypass in Mermaid foreignObject attributes

[ImChong]

Vulnerability: Found a critical DOM-based XSS vulnerability in assets/js/mermaid-config.js (sanitizeMermaidSvg) where an attacker could bypass DOMPurify by embedding malicious attributes (e.g., onload="alert(1)") in SVG foreignObject tags. The code extracted foreignObject tags using a regex, stored their unsanitized attributes, sanitized the rest of the SVG structure via DOMPurify, and then blindly re-injected the raw attributes when restoring the foreignObject content.
Impact: A malicious actor could provide specially crafted Markdown that compiles into a Mermaid graph with malicious JavaScript, bypassing the site's default DOMPurify sanitization.
Fix: Let DOMPurify natively sanitize the placeholder attributes. The regex swap was updated to attach the extracted attributes directly onto the data-fo-placeholder element before DOMPurify processes the entire SVG, effectively enforcing the sanitizer on those attributes.
Verification: Manual verification via unit test scripts that DOMPurify properly scrubs onload attributes. All pytest Python test suites complete with success.

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PR created automatically by Jules for task 3219942391315052121 started by @ImChong

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