fix: apply sidebar class for fixed width and document-editor flex layout for scrolling

frontend/src/components/DocumentEditor.tsx

@@ -96,7 +96,7 @@ export default function DocumentEditor({ documentKey, onDirtyChange, onDeleted }
   }
 
   return (
-    <div data-testid="document-editor">
+    <div className="document-editor" data-testid="document-editor">
       <div className="doc-header">
         <div className="doc-path">
           <span className="current">{documentKey}</span>

frontend/src/components/FileExplorer.tsx

@@ -26,7 +26,7 @@ export default function FileExplorer({ onSelectFile, selectedKey, refreshKey }:
   }
 
   return (
-    <div data-testid="file-explorer">
+    <div className="sidebar" data-testid="file-explorer">
       {folders.map((folder) => (
         <FolderNode key={folder} name={folder} path={folder} onSelectFile={onSelectFile} selectedKey={selectedKey} />
       ))}

frontend/src/styles.css

@@ -322,6 +322,14 @@ body {
   display: flex;
   flex-direction: column;
   min-width: 0;
+  overflow: hidden;
+}
+
+.document-editor {
+  flex: 1;
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
 }
 
 .empty-state {