feat: full shortcut parity + perf fixes across memories and duplicates

Memories view now supports the same keyboard shortcuts, heap membership,
and optimistic cache updates as the Timeline. Arrow/Ctrl+A/Escape nav is
extracted into a shared useGridKeyNav hook so both views stay in lockstep.
Duplicates view is virtualised with @tanstack/react-virtual and has
stabilised PhotoThumbnail props so React.memo actually elides work when
scrolling or toggling selection.

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

backend/app/routers/photos.py

@@ -422,6 +422,8 @@ async def get_memories(
             Photo.width,
             Photo.height,
             Photo.rating,
+            Photo.color_label,
+            Photo.is_discarded,
         )
         .where(
             Photo.user_id == current_user.id,
@@ -454,6 +456,8 @@ async def get_memories(
             "width": row.width,
             "height": row.height,
             "rating": row.rating,
+            "color_label": row.color_label,
+            "is_discarded": row.is_discarded,
         })
 
     memories = [