chore: fix type error in ObservationsList component (#280)

This is a types-only change and should have no impact on functionality.

TypeScript complained about the type we used for `<FlatList>`'s
`getItemLayout` prop. Specifically, it complained that the first
argument was invalid.

We don't even use the first argument, so we can just use the `unknown`
type to sidestep this issue.

src/frontend/screens/ObservationsList/index.tsx

@@ -32,10 +32,7 @@ const m = defineMessages({
 
 const OBSERVATION_CELL_HEIGHT = 80;
 
-const getItemLayout = (
-  data: Observation[] | null | undefined,
-  index: number,
-) => ({
+const getItemLayout = (_data: unknown, index: number) => ({
   length: OBSERVATION_CELL_HEIGHT,
   offset: OBSERVATION_CELL_HEIGHT * index,
   index,