feat(web): custom notification sound setting for thread notifications #29372

Summary

The opt-in thread notification sound (shipped in #11481) uses two hardcoded bundled assets (apps/web/src/assets/notification-completion.mp3 and notification-input.mp3). The Notifications settings only expose the mode (off / notifications / sound / notifications with sound) and the in-app toggle, so there is no way to change the chime itself.

Request

Add a per-device client setting to choose the notification sound in Settings → General → Notifications:

• Sound picker with a few bundled presets (e.g. the current ding, a bell, a soft pop) plus a "Custom file…" option that accepts a user-supplied audio file (anything Web Audio can decode: MP3/WAV/OGG), stored per device (e.g. IndexedDB) with a sensible size cap (~1–2 MB).
• Separate sounds for completion and needs-input (or at minimum one shared sound, with an optional second for input/approval).
• "Play preview" button next to the picker. Since it is an explicit user gesture, it also doubles as a Web Audio autoplay unlock.
• On decode/playback failure, fall back to the bundled default and console.warn instead of failing silently.

Why

• Preferences vary: in a noisy environment users need a louder or more distinctive chime to actually notice a completion; others prefer something soft. Today the only way to change it is to overwrite the runtime asset files on disk, which is not something most users can or should do.
• The preview button also makes the common "banner shows but no sound" problem self-diagnosable (browser autoplay blocked, muted tab, wrong output device). Right now playNotificationSound in apps/web/src/threadNotifications.ts bails with no signal when the context isn't running:

if (!audioContext || audioContext.state !== "running") return;

A user-triggerable preview would surface exactly that case.

Context

• Shipped feature: feat(web): add opt-in thread notifications and sounds #11481 (opt-in thread notifications and sounds)
• Earlier proposal that already suggested "custom sound file, plus a few bundled defaults": Add notification ding when agent needs attention #2373
• Original feature discussion: Notification sound and sidebar pulse when a thread needs input or finishes #7586