Drop observable-hooks in favor of useBehavior

With this change, views now read every reactive value through `useBehavior`, as
`architecture.md` already prescribes. Where a view reads a plain stream, the view
model now exposes a `Behavior`.

src/useBehavior.test.tsx

@@ -0,0 +1,23 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { act, renderHook } from "@testing-library/react";
+import { BehaviorSubject } from "rxjs";
+import { describe, expect, test } from "vitest";
+
+import { useBehavior } from "./useBehavior";
+
+describe("useBehavior", () => {
+  test("reads the current value and follows changes", () => {
+    const value$ = new BehaviorSubject(1);
+    const { result } = renderHook(() => useBehavior(value$));
+    expect(result.current).toBe(1);
+
+    act(() => value$.next(2));
+    expect(result.current).toBe(2);
+  });
+});