Make SVG export match GC ring controls

[happykhan]

What changed

• render SVG downloads from the same GC-filtered plot data shown on the canvas
• keep the Data download backed by the complete analysis result
• add a browser regression for disabling GC Content while leaving GC Skew enabled

Root cause

The canvas preview received plot data filtered by the GC Content and GC Skew controls, but ExportPanel regenerated SVG from the original unfiltered plot data. PNG worked because it rasterised the already-filtered live canvas.

Reproduction

The new browser test failed before the implementation change because the downloaded SVG still contained id="gc-content-ring" after GC Content was unticked.

Validation

• lint, architecture and licence checks
• 176 unit tests
• production build
• 25 production browser tests
• Vite development-mode BLAST/WASM browser test
• npm audit --omit=dev: zero vulnerabilities

The repository security script currently stops on advisory 1139427 in development-only nanoid, pulled through Vite/PostCSS. This branch does not change dependencies.

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