fix: Wrap file input in flex container for vertical centering (refs #19)

File inputs are replaced elements that ignore CSS centering properties.
The only reliable solution is to wrap the input in a flex container
with items-center.

Changes:
- Added wrapper div with `flex items-center h-11`
- Moved border/background/focus styles to the wrapper
- Input now uses flex-1 to fill available space
- Used focus-within for focus ring on wrapper

🤖 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

@@ -375,12 +375,14 @@ 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>
+          <div className="flex items-center h-11 min-h-[44px] rounded-lg border px-3 bg-white border-slate-300 focus-within:outline-none focus-within:ring-2 focus-within:ring-primary-500 dark:bg-scuro dark:border-silverstone dark:focus-within:ring-abudhabi dark:focus-within:border-abudhabi">
             <input
-            className="h-11 min-h-[44px] leading-[44px] rounded-lg border px-3 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="flex-1 text-gray-900 dark:text-avus 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 file:cursor-pointer cursor-pointer"
               type="file"
               accept="image/jpeg,image/png,application/pdf"
               onChange={(e) => setFile(e.target.files?.[0] || null)}
             />
+          </div>
           {uploadProgress > 0 && uploadProgress < 100 && (
             <div className="text-sm text-slate-600 dark:text-titanio mt-1">Uploading... {uploadProgress}%</div>
           )}