feat: add ESLint configuration and testing infrastructure

Add comprehensive ESLint setup with React and TypeScript support, create basic integration tests for the shutdown utilities, and enhance the Justfile with a new check command that runs all validation steps (cargo check, clippy, tests, and linting).

web/src/lib/api.ts

@@ -43,7 +43,7 @@ export class BannerApiClient {
       );
     }
 
-    return response.json();
+    return (await response.json()) as T;
   }
 
   async getHealth(): Promise<HealthResponse> {

web/src/reportWebVitals.ts

@@ -1,6 +1,6 @@
 const reportWebVitals = (onPerfEntry?: () => void) => {
   if (onPerfEntry && onPerfEntry instanceof Function) {
-    import('web-vitals').then(({ onCLS, onINP, onFCP, onLCP, onTTFB }) => {
+    void import('web-vitals').then(({ onCLS, onINP, onFCP, onLCP, onTTFB }) => {
       onCLS(onPerfEntry)
       onINP(onPerfEntry)
       onFCP(onPerfEntry)

web/src/routes/index.tsx

@@ -237,11 +237,11 @@ function App() {
       }
 
       // Schedule the next request after the current one completes
-      timeoutId = setTimeout(fetchData, REFRESH_INTERVAL);
+      timeoutId = setTimeout(() => void fetchData(), REFRESH_INTERVAL);
     };
 
     // Start the first request immediately
-    fetchData();
+    void fetchData();
 
     return () => {
       if (timeoutId) {