feat(web): config provider — fetch /api/config, default UI language from instance

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

web/src/main.tsx

@@ -3,6 +3,7 @@ import { createRoot } from "react-dom/client";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 
 import { App } from "./app";
+import { ConfigProvider } from "./config/config-context";
 import "./index.css";
 import "./i18n";
 
@@ -13,7 +14,9 @@ const queryClient = new QueryClient({
 createRoot(document.getElementById("root")!).render(
   <StrictMode>
     <QueryClientProvider client={queryClient}>
-      <App />
+      <ConfigProvider>
+        <App />
+      </ConfigProvider>
     </QueryClientProvider>
   </StrictMode>,
 );