fix(mermaid): inject xmlns:xlink namespace if missing before PNG export

[sleitor]

Problem

Mermaid can emit SVG containing xlink-prefixed attributes (e.g. xlink:href in C4Context and sequence diagrams) without declaring the xmlns:xlink namespace on the root <svg> element. The resulting SVG is not valid XML, so browsers refuse to load it as an image, causing PNG export to fail silently.

Minimal reproduction (C4Context diagram):

C4Context
  title Generic system context
  Person(user, "User", "Uses digital services")
  System(application, "Application", "Web and mobile service platform")

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Fix

Add addXlinkNamespaceIfMissing() in packages/streamdown/lib/mermaid/utils.ts which:

1. Checks if the SVG string contains xlink: prefixed attributes
2. Checks if xmlns:xlink namespace is already declared
3. If the namespace is missing, injects xmlns:xlink="http://www.w3.org/1999/xlink" onto the root <svg> tag via a lightweight string replacement

svgToPngBlob() now normalises the input before base64-encoding it as a data URI, following the same pattern as other Mermaid SVG normalisation fixes (see #516).

Testing

Added 6 new unit tests covering:

• Namespace injection when missing
• No modification when namespace already present
• No modification when no xlink: attributes
• Only root <svg> tag is modified (not nested)
• Various xlink attribute types (xlink:href, xlink:actuate, xlink:title)
• Integration test verifying svgToPngBlob encodes the fixed SVG

Fixes #541

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