[FEATURE]: Desktop notification/error sounds should be configurable (mute, per-event control, custom sound)

Summary

The desktop app plays the OS error notification sound for every failure (model fetch timeouts, MCP connection drops, plugin load failures, etc.) with no way to mute, lower, or change it. On an unstable network this becomes a wall of beeps — the user cannot distinguish important errors from transient blips, and there is no settings entry to silence them.

Problem

• Every error event fires the system error sound (e.g. Windows "Exclamation") through the desktop shell.
• A single minute of network flakiness produces a rapid sequence of beeps with no persistence — you cannot "look back" at what beeped.
• No global mute, no per-event-type control, no volume, no option to choose a custom sound.

Proposal

Add a Sounds/Notifications section to desktop settings:

1. Master toggle: mute all app sounds.
2. Per-event-type controls (e.g. error / notification / success) — the user decides which events are audible.
3. Optional custom sound file or OS default per event type.
4. (Complementary) a small notification toast/log line alongside each sound so audible events are also visible — pairs with the Error Viewer discussion in #41194.

Use case

Working on a flaky connection: today every failed fetch triggers a beep; with per-event settings the user could mute errors while keeping success pings, or mute everything and rely on visual indicators from #41194.

Additional context

• Desktop app (Windows), latest version
• Related: #41194 (centralized error surfacing) — sounds are the audio half of that problem