🔧 Fix TypeScript build error in API headers

✅ Issue Fixed:
- Resolved TS2322 type error in api.ts headers
- Changed from Record<string, string> to proper header merging
- Separated baseHeaders for API key injection
- Maintained compatibility with existing header options

🛠️ Technical Changes:
- Use baseHeaders for type-safe API key addition
- Merge baseHeaders with options?.headers safely
- Prevent TypeScript index signature conflicts
- Ensure build compatibility across environments

frontend/src/lib/api.ts

@@ -25,19 +25,23 @@ async function fetchApi<T>(
   retryCount = 0
 ): Promise<T> {
   // Get API key from localStorage if available
-  const apiKey = localStorage.getItem('openai_api_key');
-  const authMode = localStorage.getItem('auth_mode');
-  
-  const headers: Record<string, string> = {
+  const apiKey = localStorage.getItem("openai_api_key");
+  const authMode = localStorage.getItem("auth_mode");
+
+  const baseHeaders: Record<string, string> = {
     "Content-Type": "application/json",
-    ...options?.headers,
   };
-  
+
   // Add API key header if user is in API key mode
-  if (apiKey && authMode === 'api_key') {
-    headers["X-OpenAI-API-Key"] = apiKey;
+  if (apiKey && authMode === "api_key") {
+    baseHeaders["X-OpenAI-API-Key"] = apiKey;
   }
-  
+
+  const headers = {
+    ...baseHeaders,
+    ...options?.headers,
+  };
+
   const response = await fetch(`${API_BASE}${endpoint}`, {
     headers,
     ...options,