feat: display friendly error messages on API failure

Closes #34

Author: Ai-chan-0411

app/api/compare/route.ts

@@ -4,13 +4,38 @@ import { calculateUserScore } from "../../../lib/score";
 
 export const runtime = "nodejs";
 
+function classifyError(error: any): { message: string; status: number } {
+  const msg = error?.message ?? "";
+
+  if (msg === "User not found") {
+    return { message: "GitHub user not found. Please check the username and try again.", status: 404 };
+  }
+
+  if (msg.includes("rate limit") || error?.status === 403) {
+    return {
+      message: "GitHub API rate limit exceeded. Please wait a few minutes and try again.",
+      status: 429,
+    };
+  }
+
+  if (msg.includes("Bad credentials") || error?.status === 401) {
+    return { message: "GitHub API authentication error. Please contact the administrator.", status: 500 };
+  }
+
+  if (error?.code === "ENOTFOUND" || error?.code === "ETIMEDOUT") {
+    return { message: "Unable to reach GitHub. Please check your connection and try again.", status: 503 };
+  }
+
+  return { message: "Something went wrong while fetching data. Please try again later.", status: 500 };
+}
+
 export async function GET(request: Request) {
   const { searchParams } = new URL(request.url);
   const usernames = searchParams.getAll("username");
 
   if (usernames.length === 0) {
     return NextResponse.json(
-      { success: false, error: "provide at least one username param" },
+      { success: false, error: "Please provide at least one GitHub username." },
       { status: 400 }
     );
   }
@@ -36,13 +61,7 @@ export async function GET(request: Request) {
     return NextResponse.json({ success: true, users: results });
   } catch (error: any) {
     console.error("GitHub score error:", error);
-    const message =
-      error?.message === "User not found"
-        ? "GitHub user not found"
-        : "Failed to calculate score";
-    return NextResponse.json(
-      { success: false, error: message },
-      { status: 500 }
-    );
+    const { message, status } = classifyError(error);
+    return NextResponse.json({ success: false, error: message }, { status });
   }
 }