bug(desktop): notification sounds clip/click at playback start (need short fade-in or zero-crossing start)

### Summary

Notification/sound playback in Buzz Desktop clips at the very start of the sound - there's an audible click/pop roughly a millisecond in, before the sound properly starts. It's especially noticeable on repeated notifications and makes the notification sound feel broken/harsh.

### Environment

- App: Buzz Desktop v0.5.20 (bundle id `xyz.block.buzz.app`, macOS)
- OS: macOS 26.5.2 (Apple Silicon host running the app; also observed on Intel iMac)

### Steps to reproduce

1. Trigger any notification sound in Buzz Desktop (e.g. incoming message notification using `/sounds/boo.mp3`, `doo.mp3`, `ping.mp3` etc.)
2. Listen carefully at the very onset of the clip

### Expected

Sound should start cleanly, with either a zero-crossing start or a short fade-in (e.g. 5-10 ms) so there's no discontinuity.

### Actual

An audible transient/click a millisecond into playback - the waveform appears to start at a non-zero sample or has no attack smoothing, producing a click/pop at the start of every notification sound.

### Suggested fix

Apply a short fade-in (on the order of 5-100 ms) to the embedded notification sounds at playback, or ensure playback starts on a zero-crossing. If patching the source audio files is easier, a 0.1 s fade-in on each `/sounds/*.mp3` asset would fully resolve it.

### Extra context

I also noticed the embedded sounds appear to be stored in a compressed/proprietary container inside the desktop binary rather than as loose assets - so end users can't easily replace or fade the sounds themselves, which makes the in-app fix the only realistic path.