Perf: swap the moment.js usage for date-fns

[1nonlypiece]

Summary

There's no moment usage anywhere in the codebase to literally swap, but app/dashboard/transaction-history/page.tsx has exactly the kind of hand-rolled date arithmetic date-fns exists to replace: a custom startOfDay, manual millisecond-timestamp comparisons for date-range grouping, and toDate.setHours(23, 59, 59, 999) to compute end-of-day. Adds date-fns and swaps all of it for startOfDay/subDays/isSameDay/endOfDay/isBefore/isAfter -- same behavior, less custom date math to maintain, and isSameDay in particular is more obviously correct than comparing two millisecond timestamps for equality.

Also adds react-window as a dependency (without it, this file -- and therefore this PR's own new test -- can't even be imported; full fix, including removing the @ts-ignore/as any workaround around it, is scoped to #1581 to keep this PR's diff focused on the date-fns swap).

Testing

• node node_modules/vitest/vitest.mjs run ... tests/unit/dashboard/transaction-history-date-grouping.test.ts -- 4/4 pass, covering same-day/yesterday/older/future dates against getGroupKey
• npm run test:unit:vitest -- 301 passing (was 297 on main), zero regressions
• npx eslint app/dashboard/transaction-history/page.tsx -- clean
• npx tsc --noEmit -- no new errors in the touched file

Closes #1491