feat: create file on backend when clicking New File

- Prompt for filename before creating new file
- POST to /api/files to create the file on backend
- Set created file as currentFile and enter editing mode
- Add error handling for failed API calls

frontend/src/App.tsx

@@ -78,11 +78,26 @@ function App() {
     }
   }
 
-  const handleNewFile = () => {
-    setCurrentFile(null)
-    setContent('# New File\n\nStart writing here...')
+  const handleNewFile = async () => {
+    const filename = prompt('Enter filename:', 'untitled.md')
+    if (!filename) return
+
+    try {
+      const response = await fetch('/api/files', {
+        method: 'POST',
+        headers: {'Content-Type': 'application/json'},
+        body: JSON.stringify({filename, content: '# New File\n\nStart writing here...', title: filename.replace('.md', '')})
+      })
+      if (response.ok) {
+        const newFile = await response.json()
+        setCurrentFile(newFile)
+        setContent(newFile.content)
         setIsEditing(true)
       }
+    } catch (error) {
+      console.error('Failed to create new file:', error)
+    }
+  }
 
   const handleSave = async () => {
     if (!currentFile) {