Report Widget Client Error

Relay a widget-side thrown error to Sentry.

Uses the no_enabled_check auth dep so we still capture failures when a tenant has just toggled the widget off (the cause is often the reason for the error in the first place). Returns 204 on accepted reports so sendBeacon callers never see a non-success response; FastAPI's body validation may still return 422 for malformed payloads (e.g. missing message or over-length stack).