fix: Add bottom padding to document form grid container (refs #19)

Added pb-4 to the grid container to create consistent spacing between
the last form field (file input) and the action buttons below.

The previous approach of adding margin to a grid item didn't work as
expected due to CSS Grid layout behavior. Adding padding to the grid
container itself ensures reliable spacing.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/features/documents/components/DocumentForm.tsx

@@ -156,7 +156,7 @@ export const DocumentForm: React.FC<DocumentFormProps> = ({ onSuccess, onCancel
   return (
     <LocalizationProvider dateAdapter={AdapterDayjs}>
     <form onSubmit={handleSubmit} className="w-full">
-      <div className="grid grid-cols-1 md:grid-cols-2 gap-4">
+      <div className="grid grid-cols-1 md:grid-cols-2 gap-4 pb-4">
         <div className="flex flex-col">
           <label className="text-sm font-medium text-slate-700 dark:text-avus mb-1">Vehicle</label>
           <select