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.

[tarikbc]

Off by default was the right instinct, and documenting the audio sources with hashes is more care than most people take. That care is also what surfaced the problem.

I rebased and pushed one commit with four changes:

1. The bundled audio is gone, replaced by the macOS system sounds. Both licences you cited forbid distributing the file on a standalone basis, and a .wav committed to a public repo is exactly that, so a GPL-3.0 tree could not carry them. NSSound(named:) reaches what macOS already ships: nothing in the bundle, no licence question, and it follows the listener's output device and alert volume. Pop and Tink stand in for Bubble and Click. If you want distinctive audio later, it needs a CC0 or equivalent source that allows redistribution.

2. SOUND_LICENSES.md was shipping inside the app. XcodeGen sweeps the whole Copy directory, so it landed in Contents/Resources. I confirmed that in the built bundle. Removing the folder solved it.

3. Dropped the notification hop. copyToClipboard now calls the player directly. That also removes playPersistedSelection, which read UserDefaults.standard behind SettingsStore and duplicated the persistence contract.

4. Reverted the Shelf Size picker to .segmented. It moved to .menu in this PR without being mentioned. Unrelated changes are much easier to review on their own.

Builds clean, bundle verified free of stray resources. Merging.