fix: FILES panel should expand to fill sidebar height

- Add flex-1 to FilesPanel root div
- Sidebar is flex flex-col, FILES panel now grows to fill space
- SESSION and PROJECT panels remain natural height
- FILES panel expands into all remaining vertical space

This eliminates the gap below the FILES panel footer by making
the panel itself grow rather than just making the tree bigger.

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

@@ -10,7 +10,7 @@ export default function FilesPanel({ workspaceMode }: FilesPanelProps) {
   const isReadWrite = workspaceMode === WorkspaceMode.User;
 
   return (
-    <div className="border-t border-border-subtle flex flex-col" style={{ minHeight: '200px' }}>
+    <div className="border-t border-border-subtle flex flex-col flex-1">
       <div className="flex items-center justify-between px-4 py-2 bg-bg-tertiary">
         <h3 className="text-sm font-semibold text-text-secondary uppercase tracking-wider">
           Files