fix: FileTree scrolling and text selection issues

- Change FilesPanel overflow-hidden to overflow-auto for scrolling
- Add max-h-80 to FileTree to limit height while allowing scroll
- Add select-none to FileTreeNode to prevent text selection
- Cursor-pointer already present for clickable indication

Now the file tree scrolls independently within the FILES panel,
and text cannot be selected during rapid clicking.

gadget-code/frontend/src/components/FileTree.tsx

@@ -166,7 +166,7 @@ export default function FileTree({ workspaceMode, onFileSelect }: FileTreeProps)
   const rootLoading = state.loadingPaths.has('');
 
   return (
-    <div className="flex-1 overflow-y-auto p-2">
+    <div className="flex-1 overflow-auto p-2 max-h-80">
       {rootLoading && (
         <div className="flex items-center justify-center py-8">
           <div className="animate-spin rounded-full h-6 w-6 border-b-2 border-brand"></div>

gadget-code/frontend/src/components/FileTreeNode.tsx

@@ -72,7 +72,7 @@ export default function FileTreeNode({
   return (
     <div>
       <div
-        className={`flex items-center gap-1 px-2 py-1 rounded cursor-pointer hover:bg-bg-tertiary transition-colors ${
+        className={`flex items-center gap-1 px-2 py-1 rounded cursor-pointer hover:bg-bg-tertiary transition-colors select-none ${
           error ? 'text-red-500' : 'text-text-primary'
         }`}
         style={{ paddingLeft: `${depth * 12 + 8}px` }}

gadget-code/frontend/src/components/FilesPanel.tsx

@@ -38,7 +38,7 @@ export default function FilesPanel({ workspaceMode }: FilesPanelProps) {
           </span>
         </div>
       </div>
-      <div className="flex-1 overflow-hidden">
+      <div className="flex-1 overflow-auto">
         <FileTree
           workspaceMode={workspaceMode}
           onFileSelect={(path) => {