fix: show generated HTML charts on Win7 Chrome

Rewrite oklch/color-mix and space-separated rgb before iframe preview so pie and bar fills are not dropped on Chrome 109.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend-web/scripts/check-chrome109-css.mjs

@@ -36,6 +36,11 @@ const requiredLegacyFallbacks = [
     pattern: /htmlLabels:\s*false[\s\S]*secure:\s*\[[\s\S]*"htmlLabels"/,
     description: "Sandbox flowcharts must use native SVG labels in legacy browsers",
   },
+  {
+    file: "src/core/utils/legacy-html-preview.ts",
+    pattern: /export function downlevelModernColors[\s\S]*function injectLegacyChartShim/,
+    description: "Generated HTML charts must downlevel modern CSS colors for Chrome 109",
+  },
 ];
 
 for (const file of cssFiles) {