Perf: virtualize the transaction list over 200 rows

[1nonlypiece]

Summary

app/dashboard/transaction-history/page.tsx already had react-window-based virtualization written (FixedSizeList, TransactionVirtualRow) -- but react-window was never added to package.json and isn't in node_modules at all, so the import silently failed to resolve. It was papered over with // @ts-ignore on the import and const List = FixedSizeList as any, which suppressed the type error but not the runtime module-resolution failure -- this page could not actually build.

Adds react-window (+ @types/react-window) as real dependencies and removes the @ts-ignore/as any workarounds now that the types resolve properly (no prop mismatches -- the existing FixedSizeList usage already matched the real v1 API exactly).

Testing

• npx tsc --noEmit -- the page typechecks cleanly with react-window's real types, no new errors
• node node_modules/vitest/vitest.mjs run ... tests/unit/dashboard/transaction-history-virtualization.test.tsx -- new test renders FixedSizeList with 250 rows and asserts only a subset ever mounts into the DOM (proof it's genuinely windowing, not just sitting in a scrollable container)
• npm run test:unit:vitest -- 298 passing (was 297 on main), zero regressions
• npx eslint on both touched files -- clean

Closes #1490