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
@@ -373,7 +373,7 @@ export const DocumentForm: React.FC<DocumentFormProps> = ({ onSuccess, onCancel
           />
         </div>
 
-        <div className="flex flex-col md:col-span-2 mb-2">
+        <div className="flex flex-col md:col-span-2">
           <label className="text-sm font-medium text-slate-700 dark:text-avus mb-1">Upload image/PDF</label>
           <input
             className="h-11 min-h-[44px] rounded-lg border px-3 py-2 bg-white text-gray-900 border-slate-300 focus:outline-none focus:ring-2 focus:ring-primary-500 dark:bg-scuro dark:text-avus dark:border-silverstone dark:focus:ring-abudhabi dark:focus:border-abudhabi file:mr-4 file:py-2 file:px-4 file:rounded-md file:border-0 file:text-sm file:font-medium file:bg-primary-500/10 file:text-primary-600 dark:file:bg-abudhabi/20 dark:file:text-abudhabi"