perf: logbook memoization and pagination, debounced realtime, lazy exceljs

- Precompute per-entry spend/caffeine/sugar map; paginate ledger at 50 rows
- Debounce realtime refetches (500ms trailing) and suppress during bulk import
- Show import progress (Importing X of N)
- Dynamic-import exceljs: entry chunk 1640 -> 700 kB
- Skeleton placeholders for initial load on Overview/Trends/Logbook

src/components/Skeleton.tsx

@@ -0,0 +1,7 @@
+type SkeletonProps = {
+  className?: string;
+};
+
+export function Skeleton({ className = "" }: SkeletonProps) {
+  return <div className={className ? `skeleton ${className}` : "skeleton"} aria-hidden="true" />;
+}