fix(ui): deduplicate scheduling confirmations

[alectimison-maker]

Summary

• reconcile schedule-created broadcasts and composer confirmations by scheduled job ID
• preserve one confirmation across either event/response ordering and restored off-tab chat
• mirror the behavior in Chrome and Firefox with regression coverage

Motivation

Issue #2867 creates a single scheduled job but can display two identical confirmation messages. The scheduler already emits one creation event; the duplicate came from two UI producers: the runtime event handler and the schedule composer response.

Design

Scheduled-job confirmations now carry the canonical job ID in the DOM. A shared renderer reuses the existing confirmation regardless of whether the runtime event or composer response arrives first. Messages without a job ID keep the prior behavior.

Testing

• npm test — passed: 33 toolbar guard tests, 1,953 unit tests, offline benchmark, and 60/60 security tests
• unpacked Chrome 33.0.8 — /schedule Issue 2867 final browser check produced one job card and one confirmation, with no console or page errors

Compatibility and risks

• no scheduling, storage, alarm, or API changes
• Firefox manual UI was not run; the mirrored Firefox path is covered by the same behavioral tests

Scope

This intentionally leaves runtime message transport unchanged and makes only the presentation layer idempotent by canonical job ID.

Closes #2867

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