React 19: useRef<T>() -> useRef<T>(null) across 15 sites

React 19 dropped the no-arg useRef overload — the type-only useRef<T>()
form (no initial value) is gone, every call must pass an initial value.
The codebase had 15 occurrences of useRef<HTMLDivElement>() (DOM ref
pattern) all flagged by tsgo as 'Expected 1 arguments, but got 0'.

Mechanical sweep to useRef<HTMLDivElement>(null) — no behavior change,
React still hands out a ref object with .current set to null at mount.

Net tsgo error count: 57 -> 42.

src/common/layout/LayoutRoomPreviewerView.tsx

@@ -7,7 +7,7 @@ export const LayoutRoomPreviewerView: FC<{
 }> = props =>
 {
     const { roomPreviewer = null, height = 0 } = props;
-    const elementRef = useRef<HTMLDivElement>();
+    const elementRef = useRef<HTMLDivElement>(null);
 
     const onClick = (event: MouseEvent<HTMLDivElement>) =>
     {