fix: FILES panel should fill entire sidebar height

- Remove max-h-80 constraint from FileTree
- FileTree uses flex-1 to expand and fill available space
- Panel now properly fills sidebar from top to bottom
- File tree scrolls internally when content overflows

The FILES panel header, file tree, and footer now form a proper
flex column that fills all remaining vertical space in the sidebar.

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-auto p-2 max-h-80">
+    <div className="flex-1 overflow-auto p-2">
       {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>