feat(notifications): add MSN Messenger completion sound

[patricio-posthog]

Summary

Adds the MSN Messenger sound as a new built-in completion sound option. Users can select it under Settings → Notifications → Sound effect, and it plays when an agent finishes a task or needs input (like the other built-in sounds).

Changes

• Add msn.mp3 asset to packages/ui/src/assets/sounds/
• Register "msn" in the BuiltInCompletionSound union (settingsStore.ts)
• Wire the import + SOUND_URLS entry (sounds.ts)
• Add the MSN Messenger option to the sound dropdown (NotificationsSettings.tsx)

Testing

Ran the app locally and confirmed the sound plays via the ▶ Test sound button in Settings → Notifications, and at the selected volume.

[trunk-io]

😎 Merged directly without going through the merge queue, as the queue was empty and the PR was up to date with the target branch - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 2f4cf25.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "feat(notifications): add MSN Messenger c..." | Re-trigger Greptile}

[charlesvien]

LOOL