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.

internal/engine/scenario.go

@@ -93,6 +93,9 @@ func RunScenario(config Config, strategy Strategy) (Result, error) {
 		return Result{}, fmt.Errorf("scenario: unknown strategy %q", strategy)
 	}
 
+	if educated == nil {
+		educated = []int{} // a nil slice marshals to JSON null, not []
+	}
 	cascade := RunCascade(graph, config.Origin, config.ForwardProb, educated, thresholds)
 	return Result{
 		Strategy:       strategy,