Fix SVG blend isolation for artboards and thumbnails

[YohYamasaki]

Closes #4302

• Move the artboard rect into the same level as the artwork content because SVG's clip-path creates an isolated group, which does not use external backdrops.
• Add explicit isolation: isolate when there is no clipping by the artboard to prevent blending with the background checker pattern. The same applies to thumbnails.

[gemini-code-assist]

Code Review

This pull request introduces isolation styling to SVG outputs and artboard groups to prevent blending against the background checkerboard, and refactors artboard background rendering by moving it inside the artwork group. The review feedback points out a correctness issue where removing the x and y offset calculations breaks support for negative artboard dimensions, and provides a code suggestion to calculate local offsets relative to the group's origin.

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[cubic-dev-ai]

1 issue found across 2 files

Confidence score: 3/5

• In node-graph/libraries/rendering/src/renderer.rs, negative dimensions.x/dimensions.y can make the artboard background extend into negative space while the rect still defaults to x=0/y=0, so the background may be mispositioned or clipped for those groups; update the rect origin/size logic to account for negative dimensions before merging.

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