fix(mermaid): use XMLSerializer to produce valid SVG/XML on download

[sleitor]

Problem

Mermaid renders diagrams via innerHTML, which allows the browser to normalize elements like <br /> to their HTML void form (<br>). Saving that raw string as an SVG file fails because SVG/XML parsers reject the non-self-closing element, and PNG conversion via canvas also fails since the <img> cannot load invalid XML SVG.

Reported in #516.

Fix

Add sanitizeSvgForExport() in lib/mermaid/utils.ts that:

1. Parses the raw SVG string into the DOM via a temporary <div>.innerHTML
2. Re-serializes the <svg> element with XMLSerializer.serializeToString(), producing a well-formed XML document
3. Falls back to the original string if running outside a browser or if parsing fails

Both the SVG download path and the PNG conversion path in download-button.tsx now sanitize the SVG string before use.

Tests

• Updated existing SVG/PNG download tests to accept XMLSerializer output (which adds the XML namespace attribute)
• Added new tests for the <br> element case in both mermaid-download.test.tsx and mermaid-utils.test.ts
• All 988 tests pass

Fixes #516

[vercel]

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