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/state/TileViewModel.ts

@@ -5,10 +5,9 @@ SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 Please see LICENSE in the repository root for full details.
 */
 
-import { type Observable } from "rxjs";
-
 import { ViewModel } from "./ViewModel";
 import { type MediaViewModel, type UserMediaViewModel } from "./MediaViewModel";
+import { type Behavior } from "./Behavior";
 
 let nextId = 0;
 function createId(): string {
@@ -18,15 +17,15 @@ function createId(): string {
 export class GridTileViewModel extends ViewModel {
   public readonly id = createId();
 
-  public constructor(public readonly media$: Observable<UserMediaViewModel>) {
+  public constructor(public readonly media$: Behavior<UserMediaViewModel>) {
     super();
   }
 }
 
 export class SpotlightTileViewModel extends ViewModel {
   public constructor(
-    public readonly media$: Observable<MediaViewModel[]>,
-    public readonly maximised$: Observable<boolean>,
+    public readonly media$: Behavior<MediaViewModel[]>,
+    public readonly maximised$: Behavior<boolean>,
   ) {
     super();
   }