fix: resolve mobile file upload click issues by making native input overlay capture taps directly

frontend/src/App.css

@@ -167,6 +167,7 @@
   cursor: pointer;
   width: 100%;
   height: 100%;
+  z-index: 2;
 }
 .drop-icon {
   font-size: 2rem;

frontend/src/components/DropZone.jsx

@@ -36,7 +36,6 @@ export default function DropZone({ onFile, disabled }) {
         onDragOver={onDragOver}
         onDragLeave={onDragLeave}
         onDrop={onDrop}
-        onClick={() => !disabled && inputRef.current?.click()}
         style={disabled ? { pointerEvents: 'none', opacity: .5 } : {}}
       >
         <input