chore: upgrade React 18→19 and fix breaking type changes

- react 18.2.0 to 19.2.6
- react-dom 18.2.0 to 19.2.6
- @types/react 18.2.39 to 19.2.15
- @types/react-dom 18.2.17 to 19.2.3

React 19 breaking changes fixed:
- useRef<T>(null) now returns RefObject<T | null>; cast to
  RefObject<T> at 16 component call sites (safe, runtime unchanged)
- useRef<T>() without arg no longer valid; add | undefined>(undefined)
  in useDebounce, useFileDrop, useThrottle, useVirtualPaginator hooks,
  RoomInput, RoomTimeline, and ClientNonUIFeatures
- useReducer<typeof reducer> 1-arg form removed; drop explicit type arg
  in useForceUpdate (inferred from reducer function)
- global JSX namespace removed; import type { JSX } from react in
  react-custom-html-parser.tsx

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/hooks/useDebounce.ts

@@ -10,7 +10,7 @@ export function useDebounce<T extends unknown[]>(
   callback: DebounceCallback<T>,
   options?: DebounceOptions,
 ): DebounceCallback<T> {
-  const timeoutIdRef = useRef<number>();
+  const timeoutIdRef = useRef<number | undefined>(undefined);
   const { wait, immediate } = options ?? {};
 
   const debounceCallback = useCallback(

src/app/hooks/useFileDrop.ts

@@ -14,7 +14,7 @@ export const useFileDropZone = (
   zoneRef: RefObject<HTMLElement>,
   onDrop: (file: File[]) => void,
 ): boolean => {
-  const dragStateRef = useRef<'start' | 'leave' | 'over'>();
+  const dragStateRef = useRef<'start' | 'leave' | 'over' | undefined>(undefined);
   const [active, setActive] = useState(false);
 
   useEffect(() => {

src/app/hooks/useForceUpdate.ts

@@ -3,7 +3,7 @@ import { useReducer } from 'react';
 const reducer = (prevCount: number): number => prevCount + 1;
 
 export const useForceUpdate = (): [number, () => void] => {
-  const [state, dispatch] = useReducer<typeof reducer>(reducer, 0);
+  const [state, dispatch] = useReducer(reducer, 0);
 
   return [state, dispatch];
 };

src/app/hooks/useThrottle.ts

@@ -11,8 +11,8 @@ export function useThrottle<T extends unknown[]>(
   callback: ThrottleCallback<T>,
   options?: ThrottleOptions,
 ): ThrottleCallback<T> {
-  const timeoutIdRef = useRef<number>();
-  const argsRef = useRef<T>();
+  const timeoutIdRef = useRef<number | undefined>(undefined);
+  const argsRef = useRef<T | undefined>(undefined);
   const { wait, immediate } = options ?? {};
 
   const debounceCallback = useCallback(

src/app/hooks/useVirtualPaginator.ts

@@ -166,16 +166,22 @@ export const useVirtualPaginator = <TScrollElement extends HTMLElement>(
 
   const initialRenderRef = useRef(true);
 
-  const restoreScrollRef = useRef<{
-    scrollTop: number;
-    anchorOffsetTop: number;
-    anchorItem: number;
-  }>();
+  const restoreScrollRef = useRef<
+    | {
+        scrollTop: number;
+        anchorOffsetTop: number;
+        anchorItem: number;
+      }
+    | undefined
+  >(undefined);
 
-  const scrollToItemRef = useRef<{
-    index: number;
-    opts?: ScrollToOptions;
-  }>();
+  const scrollToItemRef = useRef<
+    | {
+        index: number;
+        opts?: ScrollToOptions;
+      }
+    | undefined
+  >(undefined);
 
   const propRef = useRef({
     range,