Perf: virtualize the transaction list over 200 rows

[greatest0fallt1me]

Note on scope

No virtualization existed anywhere in the app, and no virtualization library (react-window, react-virtual, ...) is a dependency. Added a small dependency-free hook rather than a new heavy dependency, since this app's lists have a single known fixed row height -- the simple case a ~70-line hook covers without pulling in a general-purpose library. Also opened #116 in parallel, which adds the first real consumer (`TransactionsTable`) this can wire into.

Summary

`useTransactions()` has no cap, and nothing renders it as a windowed list -- a large result set would mount one DOM row per transaction.

Change

`lib/hooks/useVirtualizedList.ts`: `useVirtualizedList(items, { rowHeight, containerHeight, overscan?, threshold? })`.

• Below `threshold` (default 200) items, it's a no-op -- returns every item, since windowing a handful of rows only adds scroll-math overhead for no benefit (matching "over 200 rows" from the issue title).
• At/above the threshold, only rows within the visible viewport (+ `overscan` on each side) are returned, with `offsetY`/`totalHeight` for the caller to position them and size the scroll container.
• The window is clamped against both ends of the list -- an out-of-range `scrollTop` (e.g. stale, or from a resize) can't push the start index past what the list can supply, which would otherwise slice to an empty result.

Tests

`lib/hooks/useVirtualizedList.test.ts`: unvirtualized below threshold, correct window at rest, window/offset shift on scroll, DOM row count stays bounded (`<=20`) even for 50,000 items, and the end-of-list clamp (scrolling far past the end still returns a valid, non-empty window ending at the last item).

`npx vitest run lib/hooks/useVirtualizedList.test.ts`: 5 passed. `npx tsc --noEmit` / `npx eslint` on changed files: clean.

Related to #117