fix: filter changes no longer blank the event list

[michaelvic123]

closes #87

Root cause

The virtual list computes its visible window using scrollTop (a React state value). When a filter is applied and the event list shrinks, the scroll reset happens in a useEffect — which runs after the render. On that first render with the new shorter list, the stale scrollTop produces a startIndex beyond the end of the filtered array, so events.slice(startIndex, endIndex) returns [] and the list goes blank.

Fix

Clamp scrollTop to the maximum valid scroll offset before computing startIndex inside the useMemo. This is a pure calculation change — one extra line — and makes the windowing logic correct regardless of effect timing.

const maxScrollTop = Math.max(0, events.length * ROW_HEIGHT - viewportHeight);
const clampedScrollTop = Math.min(scrollTop, maxScrollTop);
const startIndex = Math.max(0, Math.floor(clampedScrollTop / ROW_HEIGHT) - OVERSCAN);

Tests added

• applies a filter while scrolled deep → list must not go blank
• filter change resets scroll to top → visible rows change and remain valid

All 3 tests pass.