Add error handling and loading states

frontend/src/App.jsx

@@ -8,29 +8,66 @@ function App() {
   const [files, setFiles] = useState([]);
   const [data, setData] = useState([]);
   const [selectedFile, setSelectedFile] = useState("");
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState(null);
 
   useEffect(() => {
-    axios.get(`${API}/files`).then((res) => setFiles(res.data.files));
+    fetchFiles();
   }, []);
 
-  const loadFile = (filename) => {
+  const fetchFiles = async () => {
+    try {
+      setError(null);
+      const res = await axios.get(`${API}/files`);
+      setFiles(res.data.files);
+    } catch (err) {
+      setError("Failed to load file list. Is the backend running?");
+    }
+  };
+
+  const loadFile = async (filename) => {
     if (!filename) {
       setSelectedFile("");
       setData([]);
       return;
     }
+    setLoading(true);
+    setError(null);
     setSelectedFile(filename);
-    axios.get(`${API}/data/${filename}`).then((res) => setData(res.data));
+    try {
+      const res = await axios.get(`${API}/data/${filename}`);
+      if (Array.isArray(res.data)) {
+        setData(res.data);
+      } else if (res.data.error) {
+        setError(`File error: ${res.data.error}`);
+        setData([]);
+      } else {
+        setError("Invalid data format from server");
+        setData([]);
+      }
+    } catch (err) {
+      setError(`Failed to load data for "${filename}"`);
+      setData([]);
+    } finally {
+      setLoading(false);
+    }
   };
 
   const handleUpload = async (e) => {
     const file = e.target.files?.[0];
     if (!file) return;
-    const formData = new FormData();
-    formData.append("file", file);
-    await axios.post(`${API}/upload`, formData);
-    const res = await axios.get(`${API}/files`);
-    setFiles(res.data.files);
+    setLoading(true);
+    setError(null);
+    try {
+      const formData = new FormData();
+      formData.append("file", file);
+      await axios.post(`${API}/upload`, formData);
+      await fetchFiles();
+    } catch (err) {
+      setError("Failed to upload file");
+    } finally {
+      setLoading(false);
+    }
   };
 
   return (
@@ -40,14 +77,42 @@ function App() {
         Calories in vs. calories burned per week
       </p>
 
+      {error && (
+        <div style={{
+          marginBottom: "1rem",
+          padding: "0.75rem 1rem",
+          background: "#fee2e2",
+          color: "#dc2626",
+          borderRadius: "0.375rem",
+          border: "1px solid #fecaca"
+        }}>
+          ⚠️ {error}
+        </div>
+      )}
+
+      {loading && (
+        <div style={{ marginBottom: "1rem", color: "#6b7280" }}>
+          Loading...
+        </div>
+      )}
+
       <section style={{ marginBottom: "1.5rem" }}>
         <h2>Upload File</h2>
-        <input type="file" accept=".csv,.json" onChange={handleUpload} />
+        <input
+          type="file"
+          accept=".csv,.json"
+          onChange={handleUpload}
+          disabled={loading}
+        />
       </section>
 
       <section style={{ marginBottom: "1.5rem" }}>
         <h2>Select File</h2>
-        <select value={selectedFile} onChange={(e) => loadFile(e.target.value)}>
+        <select
+          value={selectedFile}
+          onChange={(e) => loadFile(e.target.value)}
+          disabled={loading}
+        >
           <option value="">-- Choose a file --</option>
           {files.map((f) => (
             <option key={f} value={f}>
@@ -57,7 +122,7 @@ function App() {
         </select>
       </section>
 
-      {data.length > 0 && <Chart data={data} />}
+      {data.length > 0 && !loading && <Chart data={data} />}
     </div>
   );
 }