Surface screen share failures instead of silently doing nothing

toggleScreenSharing only had `.catch(logger.error)`, so a getDisplayMedia
request that hangs (element-call-rageshakes#17152: Element Desktop on
Windows, the user pressed the screen share button 14 times in 25 seconds
and the log shows nothing but the toggle lines and livekit-client's
"waiting for pending publication promise timed out") left the user with
a button that does nothing and us with no evidence of why.

Log when a toggle is requested and when it completes or fails, with the
elapsed time, so a hang is visible in the logs. Explicit failures other
than the user cancelling the picker show as a non-modal "Could not start
screen sharing" toast. Nothing is inferred from a toggle taking a long
time: the user may simply be choosing what to share.

src/state/CallViewModel/localMember/LocalMember.test.ts

@@ -41,6 +41,7 @@ import {
   enterRTCSession,
   PublishState,
   TrackState,
+  watchScreenShareToggle,
 } from "./LocalMember";
 import {
   FailToGetOpenIdToken,
@@ -68,6 +69,31 @@ vi.mock("@livekit/components-core", () => ({
     .mockReturnValue(of({ isScreenShareEnabled: false })),
 }));
 
+describe("watchScreenShareToggle", () => {
+  it("reports nothing when the toggle completes", async () => {
+    const onError = vi.fn();
+    watchScreenShareToggle(Promise.resolve(), true, logger, onError);
+    await flushPromises();
+    expect(onError).not.toHaveBeenCalled();
+  });
+
+  it("reports failures other than the user cancelling", async () => {
+    const onError = vi.fn();
+    const e = new Error("NotReadableError");
+    watchScreenShareToggle(Promise.reject(e), true, logger, onError);
+    await flushPromises();
+    expect(onError).toHaveBeenCalledWith(e);
+  });
+
+  it("does not report the user cancelling the picker", async () => {
+    const onError = vi.fn();
+    const cancelled = new DOMException("Permission denied", "NotAllowedError");
+    watchScreenShareToggle(Promise.reject(cancelled), true, logger, onError);
+    await flushPromises();
+    expect(onError).not.toHaveBeenCalled();
+  });
+});
+
 describe("LocalMembership", () => {
   describe("enterRTCSession", () => {
     it("It joins the correct Session", () => {