Currently, recordings are saved to a hardcoded directory in SettingsStore.outputDirectory (~/Documents/BetterCapture). Users need to be able to choose a custom folder.

Implementation Hints:

• Add a persisted property 'customOutputDirectory: String?' to SettingsStore (store the bookmark data or path).
• Update SettingsStore.outputDirectory to return the custom path if set.
• In SettingsView, add a row showing the current path and a 'Change...' button.
• Use NSOpenPanel to select a directory.