fix(duplicates): reserve 160px row height so thumbnails don't shift on load

The group grid only pinned column width; rows defaulted to auto height,
so each cell collapsed to the size of its still-empty <img> and snapped
to 160px once the thumbnail arrived — visible layout jump, plus the
virtualizer re-measured every group on image load.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/duplicates/DuplicatesView.tsx

@@ -386,8 +386,12 @@ const DuplicateGroupSection = memo(function DuplicateGroupSection({
         ref={gridRef}
         className="grid gap-1 p-2"
         style={{
-          gridTemplateColumns:
-            'repeat(auto-fill, 160px)',
+          // Fix both axes so each cell reserves a 160×160 box before
+          // its thumbnail finishes loading — otherwise rows collapse to
+          // the height of an empty <img> and jump when images arrive,
+          // which also throws off the virtualizer's row-height measure.
+          gridTemplateColumns: 'repeat(auto-fill, 160px)',
+          gridAutoRows: '160px',
         }}
       >
         {group.members.map((member) => {