fix(svg): support font-family attribute on SVG text elements

[hjl12345]

Description

Modified getUsedFonts function to traverse SVGElement child nodes, and added test cases for SVG text elements with font-family attribute.

Motivation and Context

This PR fixes the issue reported in #534.

When using toPng to export an SVG containing <text> elements with custom fonts (e.g., font-family="zihunlieridangkongti"), the exported PNG does not apply the custom font. The root cause is that getUsedFonts only traverses HTMLElement and ignores SVGElement nodes, so the font-family attribute on SVG text elements is never collected.

This change ensures that fonts set via SVG font-family attribute are properly collected and embedded during the export process.

Closes #534

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Enhancement (changes that improvement of current feature or performance)
• Refactoring (changes that neither fixes a bug nor adds a feature)
• Test Case (changes that add missing tests or correct existing tests)
• Code style optimization (changes that do not affect the meaning of the code)
• Docs (changes that only update documentation)
• Chore (changes that don't modify src or test files)

Self Check before Merge

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[biiibooo]

👋 @hjl12345

💖 Thanks for opening this pull request! 💖

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.50%. Comparing base (d9b2fcf) to head (9ee9e0f).
⚠️ Report is 26 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #581   +/-   ##
=======================================
  Coverage   66.50%   66.50%           
=======================================
  Files          10       10           
  Lines         612      612           
  Branches      150      150           
=======================================
  Hits          407      407           
  Misses        144      144           
  Partials       61       61           

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[hjl12345]

@bubkoo I know you're busy, but could you please review this PR when you have a moment? It fixes the SVG font issue introduced by commit 09bee44. Thank you!