fix: resolve CSS custom properties (var()) in SVG descendants

[Sunyi-000]

Problem

Since v1.11.12, CSS custom properties (var()) defined in external stylesheets no longer appear in exported images when used inside SVG elements (e.g., Recharts with MUI theme colors).

Root cause: PR #462 introduced a performance optimization that deep-clones SVG elements using cloneNode(true). The early return added to cloneChildren for SVG nodes means cloneCSSStyle() is never called on SVG child elements. Since cloneCSSStyle uses window.getComputedStyle() to resolve var() references to their actual values, skipping it leaves CSS variables unresolved. When the SVG is serialized, those variable definitions are unavailable in the export context, so colors render as empty/transparent.

Reproducible example: https://codesandbox.io/p/sandbox/vq6ml5

Fixes #500

Solution

After deep-cloning the SVG, iterate over all native/cloned descendant pairs and call cloneCSSStyle() on each. getComputedStyle() resolves var(--name) to its actual computed value, which then gets inlined on the cloned element.

This preserves the performance benefit of the deep-clone optimization (no async per-child cloneNode calls for the SVG subtree) while correctly resolving CSS variables.

Why not PR #521?

PR #521 simply removes the early return for SVG elements, which causes SVG children to be re-appended to an already-populated deep-cloned SVG, resulting in duplicate children. This fix avoids that by only copying styles, not re-cloning the subtree.

Changes

• src/clone-node.ts: 17 lines changed in cloneChildren

[biiibooo]

👋 @Sunyi-000

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[codecov]

Codecov Report

❌ Patch coverage is 30.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.50%. Comparing base (d9b2fcf) to head (a8def89).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
src/clone-node.ts	30.00%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #575   +/-   ##
=======================================
  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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