dsh-audio-alert: Browser alert sounds for approval, inquiry, and completion events on dsh web

`dsh-audio-alert` is a lightweight DSH plugin designed for `dsh web`, aimed at addressing the issue where users easily miss task status changes when they are not continuously monitoring the tab. When the plugin detects three key types of events requiring user attention—approval requests, user questions, and turn endings—it automatically plays alert sounds in the browser, thereby converting visual waiting states into audible audio alerts. The plugin supports fine-grained configuration via the `audio-alert` option in `settings.yaml`, including an enable/disable toggle, volume adjustment, and independent sound source configuration for different event types (supporting data URI and HTTP URL).