feat: add new completion sound effects

[christiaan-ph]

Problem

Limited selection of completion sounds. More variety gives users better options to find a sound they like.

Changes

• Added 7 new completion sound mp3 files: bubbles, drop, knock, ring, shoot, slide, switch
• Added the new sounds to the CompletionSound type, sound URL mapping, and settings dropdown

Also FYI. This is the source of the sounds: https://kenney.nl/assets/interface-sounds

They are all CC0 1.0 Universal License

How did you test this?

Tested manually following these steps:

1. Open Settings > General
2. Find the "Completion sound" dropdown
3. Select each new sound (Bubbles, Drop, Knock, Ring, Shoot, Slide, Switch)
4. Click "Test" to preview each one
5. Run a task to completion and verify the selected sound plays

[adboio]

i haven't listened to this but i trust you 😛 thanks!!