fix(calls): no "the shared window was closed" toast when an admin turns sharing off (#297)

When a room admin turned off Allow Screen Sharing, the sharer got two
toasts: the correct "turned off by a room admin" and the fork's
"ended" notice, "the shared window was closed", which was wrong. Skip
the ended notice while the room policy forbids sharing.

Fixes #297

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/hooks/useScreenshareNotices.test.ts

@@ -0,0 +1,10 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { shouldShowScreenshareNotice } from './useScreenshareNotices';
+
+test('shouldShowScreenshareNotice: no "window was closed" when an admin turned sharing off (#297)', () => {
+  assert.equal(shouldShowScreenshareNotice('ended', false), false);
+  assert.equal(shouldShowScreenshareNotice('ended', true), true);
+  assert.equal(shouldShowScreenshareNotice('no-frames', false), true);
+  assert.equal(shouldShowScreenshareNotice('alone', true), true);
+});