web: parity page renders the three-scenario comparison from live data

App.vue fetches the default config, posts it to /api/comparison, and
renders ComparisonTable; the Vite dev server proxies /api to the Go
server so the browser sees one origin. src/lib/api.ts is the only
fetch code and uses exclusively generated types: no shape is defined
on the frontend. Scaffold example components removed.
Engine fix surfaced by the smoke test: a nil Go slice marshals to
JSON null, violating the generated 'educated: number[]' contract;
RunScenario now returns an empty slice instead.
Verified end to end: curl through the Vite proxy returns the golden
99/70/7. Vitest covers the table rendering; type-check, oxlint,
eslint clean. This completes the milestone 2 parity check.

web/vite.config.ts

@@ -15,4 +15,11 @@ export default defineConfig({
       '@': fileURLToPath(new URL('./src', import.meta.url))
     },
   },
+  server: {
+    // The Go server owns /api; run it next to `npm run dev` with:
+    //   go run ./cmd/spreadlab
+    proxy: {
+      '/api': 'http://localhost:8080',
+    },
+  },
 })