feat: add other-user device sessions list with per-device verification

Adds a collapsible "Sessions" section to the user profile card that
appears when cross-signing is active and the profile belongs to another
user. Each session shows a colour-coded shield (green = verified, yellow
= unverified) and a "Verify" button for unverified devices that
initiates the SAS emoji flow via crypto.requestDeviceVerification.

New hook useOtherUserDevices fetches the target user's device list via
crypto.getUserDeviceInfo and reacts to CryptoEvent.DevicesUpdated.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/hooks/useOtherUserDevices.ts

@@ -0,0 +1,54 @@
+import { useCallback, useEffect, useState } from 'react';
+import { useMatrixClient } from './useMatrixClient';
+import { useCrossSigningActive } from './useCrossSigning';
+import { useDeviceListChange } from './useDeviceList';
+
+export type UserDevice = {
+  deviceId: string;
+  displayName?: string;
+};
+
+export function useOtherUserDevices(userId: string): UserDevice[] | undefined {
+  const mx = useMatrixClient();
+  const crossSigningActive = useCrossSigningActive();
+  const [devices, setDevices] = useState<UserDevice[] | undefined>(undefined);
+
+  const fetchDevices = useCallback(async () => {
+    const crypto = mx.getCrypto();
+    if (!crypto || !crossSigningActive) {
+      setDevices(undefined);
+      return;
+    }
+    try {
+      const deviceMap = await crypto.getUserDeviceInfo([userId], true);
+      const userDevices = deviceMap.get(userId);
+      if (!userDevices) {
+        setDevices([]);
+        return;
+      }
+      setDevices(
+        Array.from(userDevices.values()).map((device) => ({
+          deviceId: device.deviceId,
+          displayName: device.displayName,
+        })),
+      );
+    } catch {
+      setDevices(undefined);
+    }
+  }, [mx, userId, crossSigningActive]);
+
+  useEffect(() => {
+    fetchDevices();
+  }, [fetchDevices]);
+
+  useDeviceListChange(
+    useCallback(
+      (userIds: string[]) => {
+        if (userIds.includes(userId)) fetchDevices();
+      },
+      [userId, fetchDevices],
+    ),
+  );
+
+  return devices;
+}