fix(notifications): dedupe repeated presentation

[k11kirky]

Problem

Repeated delivery of the same app notification can play the completion sound and show its toast twice.

Why: notification delivery is at-least-once, so presentation needs to be idempotent during a short replay window.

Changes

Deduplicate identical, non-suppressed notifications for one second before triggering sound, toast, native notification, or dock effects. Added a regression test covering duplicate completion delivery.

How did you test this?

• Red/green focused notification test (21 tests passing)
• Biome lint on both changed files
• git diff --check

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 786d222.}