Replace many usages of useObservableEagerState with useBehavior

This hook is simpler in its implementation (therefore hopefully more correct & performant) and enforces a type-level distinction between raw Observables and Behaviors.

src/room/ReactionsOverlay.tsx

@@ -6,16 +6,16 @@ Please see LICENSE in the repository root for full details.
 */
 
 import { type ReactNode } from "react";
-import { useObservableState } from "observable-hooks";
 
 import styles from "./ReactionsOverlay.module.css";
 import { type CallViewModel } from "../state/CallViewModel";
+import { useBehavior } from "../useBehavior";
 
 export function ReactionsOverlay({ vm }: { vm: CallViewModel }): ReactNode {
-  const reactionsIcons = useObservableState(vm.visibleReactions$);
+  const reactionsIcons = useBehavior(vm.visibleReactions$);
   return (
     <div className={styles.container}>
-      {reactionsIcons?.map(({ sender, emoji, startX }) => (
+      {reactionsIcons.map(({ sender, emoji, startX }) => (
         <span
           // Reactions effects are considered presentation elements. The reaction
           // is also present on the sender's tile, which assistive technology can