chore(deps): upgrade react to 19.2.7 and react types to 19.x (#1154)

Bump react and react-dom 19.1.2 -> 19.2.7 and @types/react and
@types/react-dom from v18 to v19. Keeps babel-plugin-react-compiler,
which is a separate build-time tool, not part of the React 19 runtime.

The @types v19 upgrade surfaced 21 type-only errors, all fixed without
runtime behavior changes:

- useRef<T>() now requires an explicit argument: pass undefined
- DOM element refs are RefObject<T | null>: widen PostFormFields and
  AdvancedSettings ref prop types accordingly
- retype reply-modal onInput handler to FormEvent + currentTarget
- add a null guard for the boards-bar search input
- update test mocks for stricter cloneElement/forwardRef typings

Verified: type-check, lint, 1051 tests, and build pass; react-doctor
score unchanged at 54; browser smoke across Chromium/Firefox/WebKit
plus mobile viewport clean.

src/views/post/post.tsx

@@ -298,7 +298,7 @@ const PostPage = () => {
   const communityAddress = resolvedCommunityAddress ?? commentCommunityAddress;
   const communityIdentifier = useCommunityIdentifier(communityAddress);
   const consumedThreadTopScrollRef = useRef<string | null>(null);
-  const previousThreadCidRef = useRef<string>();
+  const previousThreadCidRef = useRef<string>(undefined);
   const lastProcessedUpdateRequestIdRef = useRef(0);
 
   const navigate = useNavigate();