Add optional clipboard capture sounds

[brzvsk]

What changed

• Adds a compact Copy Sound dropdown in General settings with Off, Bubble, and Click
• Keeps sound disabled by default and previews a sound when it is selected
• Plays the selected sound only after a clipboard item is successfully saved
• Also supports successful shelf Cmd+C from Add standard shelf keyboard actions #1 without re-ingesting or duplicating the marked pasteboard write
• Bundles short, loudness-matched WAV assets and records their sources, licenses, hashes, and processing notes

Testing

• swift test --package-path CopyCore --disable-automatic-resolution — 184 tests passed
• Debug app target builds successfully with Xcode 26.6
• Installed and launched a locally signed combined build with Add standard shelf keyboard actions #1 and Improve shelf image previews and timestamp layout #2 using the existing bundle identifier
• Verified the final bundle contains only the Bubble and Click WAV resources

Integration note

This PR and #1 build independently. A small successful-copy notification added to #1 connects shelf Cmd+C to this optional feedback while preserving the clipboard monitor own-write suppression.