fix: reduce main layout height offset from 180px to 120px across all tools

Also restore scroll handling to ExportPanel and PresetLibrary, and
remove maxHeight cap from CodeSnippet in ExportPanel.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/calculate/Calculator.tsx

@@ -34,7 +34,7 @@ export default function Calculator() {
       {/* Main layout — side-by-side on lg, tabbed on mobile */}
       <div
         className="grid grid-cols-1 lg:grid-cols-5 gap-4"
-        style={{ height: 'calc(100svh - 180px)'  }}
+        style={{ height: 'calc(100svh - 120px)'  }}
       >
         {/* Expression panel */}
         <div