Support SVG gradient and pattern paints

[mezotv]

Problem

Kiwi currently ignores SVG paint values that use url(...), which drops gradients and patterns.

Impact

Gradient logos and illustrated SVG assets import with missing or incorrect fills.

Suggested scope

• Detect linearGradient, radialGradient, and basic pattern references from fill/stroke.
• Decide whether to map gradients to Figma paints or provide a graceful fallback.
• Add examples that verify unsupported paints do not silently become black shapes.

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Linear issue: NOT-74