fix(admin): full-width ext selector in file viewer templates (#3361)

The default-opening-method extension DenseSelect in the viewer edit
dialog did not stretch to its cell width, unlike the sibling fields.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

frontend/src/component/Admin/FileSystem/ViewerSetting/FileViewerEditDialog.tsx

@@ -170,7 +170,7 @@ function DraggableTemplateRow({
       hover
     >
       <NoWrapTableCell>
-        <DenseSelect value={template.ext} required onChange={onExtChange}>
+        <DenseSelect value={template.ext} required onChange={onExtChange} fullWidth>
           {extList.map((ext) => (
             <SquareMenuItem value={ext} key={ext}>
               <ListItemText slotProps={{ primary: { variant: "body2" } }}>{ext}</ListItemText>