#363 Choose the notification sound

[Anonymous]

Originally created by: kristofferR

Problem

notification_sound is on/off only (settings.rs:104). People want to choose the sound (Caprine: [sindresorhus/caprine#201](https://github.com/sindresorhus/caprine/issues/201)).

Design

• Replace the toggle with a picker: Off, System default, a few bundled sounds, or "Choose file…" (wav/aiff/mp3/ogg depending on platform).
• Deliver the sound through each platform's native notification sound where possible (macOS UNNotificationSound named sound copied into ~/Library/Sounds, Windows toast audio, Linux sound-file hint), with a "Preview" button.
• Platform limits (details in [#446]): Windows toasts can only play the built-in ms-winsoundevent sounds for apps installed without a package, as Chatpane is. A bundled or chosen file there means a silent toast with Chatpane playing the file itself. On Linux many daemons ignore sound-file and sound-name, so Chatpane may need to play the sound itself there too. [#446] also lists each OS's own sounds in the picker.
• Migrate the existing boolean: true → System default, false → Off.

Testing

Each option on each platform, with notifications grouped and ungrouped; Off stays silent; the migration preserves current behaviour.

Part of [#349].