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

[demarant]

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

[santiagorodriguezbermudez]

+1 from me. I run several threads at once and want to tell my agents apart from everything else on my Mac by ear. Being able to pick the sound for "finished" and "needs input" (or drop in my own file) would help a lot.