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/settings/DeviceSelection.test.tsx

@@ -0,0 +1,82 @@
+/*
+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 { render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { Form } from "@vector-im/compound-web";
+import { describe, expect, it, vi } from "vitest";
+
+import { DeviceSelection } from "./DeviceSelection";
+import { constant } from "../state/Behavior";
+import {
+  type AudioOutputDeviceLabel,
+  type DeviceLabel,
+  type MediaDevice,
+  type SelectedDevice,
+} from "../state/MediaDevices";
+
+type Device = MediaDevice<DeviceLabel | AudioOutputDeviceLabel, SelectedDevice>;
+
+function mockDevice(
+  available: [string, DeviceLabel][],
+  selectedId: string | undefined,
+): Device & { select: ReturnType<typeof vi.fn> } {
+  return {
+    available$: constant(new Map(available)),
+    selected$: constant(
+      selectedId === undefined ? undefined : { id: selectedId },
+    ),
+    select: vi.fn(),
+  } as unknown as Device & { select: ReturnType<typeof vi.fn> };
+}
+
+const twoDevices: [string, DeviceLabel][] = [
+  ["a", { type: "name", name: "Mic A" }],
+  ["b", { type: "number", number: 2 }],
+];
+
+describe("DeviceSelection", () => {
+  it("marks the selected device and selects another on click", async () => {
+    const device = mockDevice(twoDevices, "a");
+    render(
+      <Form.Root>
+        <DeviceSelection
+          device={device}
+          title="Microphone"
+          numberedLabel={(n) => `Microphone ${n}`}
+        />
+      </Form.Root>,
+    );
+
+    const first = screen.getByRole<HTMLInputElement>("radio", {
+      name: "Mic A",
+    });
+    const second = screen.getByRole<HTMLInputElement>("radio", {
+      name: "Microphone 2",
+    });
+    expect(first.checked).toBe(true);
+    expect(second.checked).toBe(false);
+
+    await userEvent.click(second);
+    expect(device.select).toHaveBeenCalledWith("b");
+  });
+
+  it("shows nothing when there is no choice to make", () => {
+    const device = mockDevice([twoDevices[0]], "a");
+    render(
+      <Form.Root>
+        <DeviceSelection
+          device={device}
+          title="Microphone"
+          numberedLabel={(n) => `Microphone ${n}`}
+        />
+      </Form.Root>,
+    );
+    expect(screen.queryByRole("radio")).toBeNull();
+    expect(screen.queryByText("Microphone")).toBeNull();
+  });
+});

src/settings/DeviceSelection.tsx

@@ -21,7 +21,6 @@ import {
   Separator,
 } from "@vector-im/compound-web";
 import { Trans, useTranslation } from "react-i18next";
-import { useObservableEagerState } from "observable-hooks";
 
 import {
   type AudioOutputDeviceLabel,
@@ -30,6 +29,7 @@ import {
   type MediaDevice,
 } from "../state/MediaDevices";
 import styles from "./DeviceSelection.module.css";
+import { useBehavior } from "../useBehavior";
 
 interface Props {
   device: MediaDevice<DeviceLabel | AudioOutputDeviceLabel, SelectedDevice>;
@@ -44,8 +44,8 @@ export const DeviceSelection: FC<Props> = ({
 }) => {
   const { t } = useTranslation();
   const groupId = useId();
-  const available = useObservableEagerState(device.available$);
-  const selectedId = useObservableEagerState(device.selected$)?.id;
+  const available = useBehavior(device.available$);
+  const selectedId = useBehavior(device.selected$)?.id;
   const onChange = useCallback(
     (e: ChangeEvent<HTMLInputElement>) => {
       device.select(e.target.value);