feat(notifications): scale completion sound speed with task length

[frankh]

Summary

Adds an opt-in "Scale sound speed with task length" toggle to Settings → Notifications. When enabled, the completion sound's playback rate reflects how long the just-completed turn took:

• ≤ 30s → ~3× faster (higher pitch)
• 2–4 min → normal speed (the "normal" band)
• ≥ 30 min → ~3× slower (lower pitch)
• smooth log-interpolation across the two ramps, clamped to [1/3, 3]

Off by default, so existing behavior is unchanged unless the user turns it on. It uses HTMLAudioElement.playbackRate, so faster also means higher-pitched and slower means lower-pitched.

How it works

• New playbackRateForTaskDuration(durationMs) helper in sounds.ts; playCompletionSound gains an optional playbackRate param.
• The just-completed turn duration is computed in sessionService from acpMsg.ts - session.promptStartedAt (Immer keeps the pre-update snapshot, so the start time is still readable), plumbed through notifyPromptComplete → NotificationBus.notify → playCompletionSound.
• New scaleSoundWithTaskLength setting added to the settings store, the NotificationSettings contract, the desktop settings provider, and the Notifications settings UI (shown only when a sound is selected; included in "Reset to defaults").

Tests

• sounds.test.ts: parametrized coverage of the duration→rate mapping across anchor/boundary points.
• notifications.test.ts: asserts the scaled rate is applied when the toggle is on with a duration, and 1 when off or when no duration is available.

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Created with PostHog Code

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit ef02594.}

[pauldambra]

do more weird

[greptile-apps]

_{Reviews (1): Last reviewed commit: "feat(notifications): scale completion so..." | Re-trigger Greptile}