feat: add system reset page and centralize API utility functions

frontend/src/app/reset/page.tsx

@@ -25,7 +25,7 @@ export default function ResetPage() {
     setResult(null);
 
     try {
-      const baseUrl = process.env.NEXT_PUBLIC_API_URL || "http://localhost:8000";
+      const baseUrl = process.env.NEXT_PUBLIC_API_URL || "";
       const resp = await fetch(`${baseUrl}/api/admin/reset-db?password=${encodeURIComponent(password)}`, {
         method: "POST",
       });

frontend/src/lib/api.ts

@@ -1,6 +1,6 @@
 const API_BASE = typeof process.env.NEXT_PUBLIC_API_URL === "string" 
   ? process.env.NEXT_PUBLIC_API_URL 
-  : "http://localhost:8000";
+  : ""; // Fallback to relative paths for single-origin deployments (HF/Nginx)
 
 async function request<T>(path: string, options?: RequestInit): Promise<T> {
   const url = path.startsWith("http") ? path : `${API_BASE}${path}`;