feat(proxy): add outbound HTTP proxy support for OAuth + provider requests

- Patch Node fetch via undici ProxyAgent when HTTP_PROXY/HTTPS_PROXY/ALL_PROXY is set
- Ensure proxy patch is loaded for both chat pipeline and OAuth token exchange
- Add Dashboard Settings → Network to edit outbound proxy and apply immediately
- Persist outbound proxy settings in local db and initialize on server startup
- Move proxy helpers to src/lib/network/ for better structure
- Rename src/proxy.js → src/dashboardGuard.js to avoid naming confusion
- Re-apply proxy env after DB import
- Fix: close old dispatcher on proxy URL change to prevent connection pool leak
- Fix: idempotency guard to avoid patching globalThis.fetch multiple times

Made-with: Cursor

src/app/layout.js

@@ -2,6 +2,7 @@ import { Inter } from "next/font/google";
 import "./globals.css";
 import { ThemeProvider } from "@/shared/components/ThemeProvider";
 import "@/lib/initCloudSync"; // Auto-initialize cloud sync
+import "@/lib/network/initOutboundProxy"; // Auto-initialize outbound proxy env
 
 const inter = Inter({
   subsets: ["latin"],