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/lib/excel.ts

@@ -1,4 +1,4 @@
-import ExcelJS from "exceljs";
+import type ExcelJS from "exceljs";
 import { flavourMeta } from "../data/flavours";
 import type { EntryDraft, ImportPreview, ImportPreviewRow, RedBullEntry } from "../types";
 import {
@@ -39,6 +39,7 @@ const ENTRY_COLUMNS = [
 ] as const;
 
 export async function createExcelExport(entries: RedBullEntry[]) {
+  const { default: ExcelJS } = await import("exceljs");
   const workbook = new ExcelJS.Workbook();
   workbook.creator = "Red Bull Intake Tracker";
   workbook.created = new Date();
@@ -53,6 +54,7 @@ export async function createExcelExport(entries: RedBullEntry[]) {
 }
 
 export async function parseExcelImport(file: File, existingEntries: RedBullEntry[]): Promise<ImportPreview> {
+  const { default: ExcelJS } = await import("exceljs");
   const workbook = new ExcelJS.Workbook();
   await workbook.xlsx.load(await file.arrayBuffer());