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() without arg no longer valid; add | undefined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/components/CallEmbedProvider.tsx

@@ -399,7 +399,7 @@ type CallEmbedProviderProps = {
 };
 export function CallEmbedProvider({ children }: CallEmbedProviderProps) {
   const callEmbed = useAtomValue(callEmbedAtom);
-  const callEmbedRef = useRef<HTMLDivElement>(null);
+  const callEmbedRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
   const joined = useCallJoined(callEmbed);
 
   const selectedRoom = useSelectedRoom();

src/app/components/ConfirmPasswordMatch.tsx

@@ -12,8 +12,8 @@ type ConfirmPasswordMatchProps = {
 };
 export function ConfirmPasswordMatch({ initialValue, children }: ConfirmPasswordMatchProps) {
   const [match, setMatch] = useState(initialValue);
-  const passRef = useRef<HTMLInputElement>(null);
-  const confPassRef = useRef<HTMLInputElement>(null);
+  const passRef = useRef<HTMLInputElement>(null) as React.RefObject<HTMLInputElement>;
+  const confPassRef = useRef<HTMLInputElement>(null) as React.RefObject<HTMLInputElement>;
 
   const doMatch = useDebounce(
     useCallback(() => {

src/app/components/emoji-board/EmojiBoard.tsx

@@ -421,8 +421,8 @@ export function EmojiBoard({
     { wait: 200 },
   );
 
-  const contentScrollRef = useRef<HTMLDivElement>(null);
-  const virtualBaseRef = useRef<HTMLDivElement>(null);
+  const contentScrollRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
+  const virtualBaseRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
   const virtualizer = useVirtualizer({
     count: groups.length,
     getScrollElement: () => contentScrollRef.current,