fix: Vertically center file input content (refs #19)

The "Choose File" button and "No file chosen" text were not vertically
centered within the file input. This was caused by:
1. Browser default `align-items: baseline` for file inputs
2. Conflicting `py-2` padding on the input container

Fixed by:
- Removing `py-2` (conflicting vertical padding)
- Adding `flex items-center` (explicit vertical centering)

🤖 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

@@ -376,7 +376,7 @@ export const DocumentForm: React.FC<DocumentFormProps> = ({ onSuccess, onCancel
         <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"
+            className="h-11 min-h-[44px] rounded-lg border px-3 flex items-center 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"
             type="file"
             accept="image/jpeg,image/png,application/pdf"
             onChange={(e) => setFile(e.target.files?.[0] || null)}