🐛(frontend) stop reporting screen-share denials as errors

Add a small helper that classifies a `getDisplayMedia` failure as a
user, browser, or OS permission denial, or returns null when it is
a genuine error.

Chromium reports denials with explicit, non-localized messages:

* "Permission denied by user" when the user cancels or dismisses
  the source picker.
* "Permission denied by system" when the OS blocks capture (e.g.
  the macOS Screen Recording privacy setting).
* Plain "Permission denied" for browser-level blocks (site
  settings, enterprise policy, permissions-policy).

Firefox and Safari use generic `NotAllowedError` messages, which
fall into the "browser" bucket.

Firefox additionally does not map macOS Screen Recording (TCC)
blocks to `NotAllowedError`: the OS silently returns no capturable
sources, so `getDisplayMedia` rejects with `NotFoundError` ("The
object can not be found here."). Same quirk as the mic/cam OS blocks
handled in `useWatchMediaDeviceErrors` via `isLikelySystemNotFound`.

Behavior on a denied screen-share permission:

* Denials are expected outcomes (picker cancelled by the user, OS
  privacy settings, enterprise policy…) and no longer surface as
  exceptions in error tracking; capture an analytics event instead.
* Only OS-level blocks get the modal, since it explains how to
  unblock them.

src/frontend/src/features/analytics/telemetry.ts

@@ -137,6 +137,7 @@ export const captureMediaEvent = async (
     | 'media-device-success'
     | 'device-not-found'
     | 'permissions-denied'
+    | 'screen-share-permission-denied'
     | 'silent-mic-detected'
     | 'silent-mic-analyser-unavailable'
     | 'silent-mic-recovered'