ui: uniform 160px thumbnail cells across all grid views

Timeline, Memories, and Duplicates now share a single fixed cell size
(THUMBNAIL_SIZE=160) with no 1fr stretching — cells stay exactly 160px
regardless of sidebar state, at the cost of a small right-edge strip
when the container width isn't a multiple of (160+gap). Width is
measured on the scroll container itself with padding subtracted so
sidebar expand/collapse reliably reflows the grid.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/duplicates/DuplicatesView.tsx

@@ -87,7 +87,7 @@ export function DuplicatesView() {
 
   // Track the rendered column count of the duplicates grid so ↑/↓ can
   // skip a row instead of jumping a single cell. The grid uses
-  // `repeat(auto-fill, minmax(180px, 1fr))` so columns = floor(width/180).
+  // `repeat(auto-fill, minmax(160px, 1fr))` so columns = floor(width/160).
   // We measure the FIRST section's grid container — every section uses
   // the same auto-fill rule so any one is representative.
   const [columns, setColumns] = useState(4)
@@ -100,7 +100,7 @@ export function DuplicatesView() {
     sampleObserverRef.current = null
     if (!el) return
     const measure = () => {
-      const cols = Math.max(1, Math.floor(el.clientWidth / 180))
+      const cols = Math.max(1, Math.floor(el.clientWidth / 160))
       setColumns(cols)
     }
     measure()
@@ -386,7 +386,7 @@ const DuplicateGroupSection = memo(function DuplicateGroupSection({
         className="grid gap-1 p-2"
         style={{
           gridTemplateColumns:
-            'repeat(auto-fill, minmax(180px, 1fr))',
+            'repeat(auto-fill, 160px)',
         }}
       >
         {group.members.map((member) => {
@@ -399,7 +399,7 @@ const DuplicateGroupSection = memo(function DuplicateGroupSection({
             >
               <PhotoThumbnail
                 photo={photoByMemberId.get(member.id)!}
-                size={180}
+                size={160}
                 fill
                 isSelected={selectedSet.has(member.id)}
                 onClick={handleClick}