fix(finder): observe mounted volumes and their lifecycle to support external/network drives (fixes #148, #150)

[Juns-g]

Summary

Fixes #148
Fixes #150

Currently, setupObservingDirectories() in FinderSyncExt only registers ["/"]. In macOS's VFS architecture, external drives, mounted DMGs, and network shares (SMB/NFS/AFP) reside under /Volumes/<VolumeName>, which are separate file system mount points not covered by root volume-only observation in FinderSync. As a result, right-click context menus fail to appear on external or network volumes.

This PR fixes this by:

1. Explicitly registering both / and each currently mounted volume (FileManager.default.mountedVolumeURLs).
2. Adding MountedVolumeObserver to dynamically listen to NSWorkspace.didMountNotification, didUnmountNotification, and didRenameVolumeNotification on NSWorkspace.shared.notificationCenter.
3. Updating FIFinderSyncController.default().directoryURLs incrementally to prevent stale references when drives are mounted/unmounted at runtime.

Type of Change

• 🐛 Bug fix
• 🧪 Test addition/update
• 📝 Documentation update

Related Issue

Closes #148
Closes #150

Testing

Manual Testing

• Tested with real mounted DMG: dynamically added to observation list upon mount and pruned on unmount.
• Verified FinderSync extension loads properly with updated directory URLs.

Build Verification

xcodebuild -project RClick.xcodeproj -scheme RClick -destination 'platform=macOS'
xcodebuild test -project RClick.xcodeproj -scheme RClick -destination 'platform=macOS'

All unit tests pass including the new MountedVolumeObserverTests.

Checklist

• My code follows the project's Swift 6.2 conventions (see CONTRIBUTING.md)
• All UI is SwiftUI (no AppKit UI components)
• I have tested on macOS 15.6+
• I have updated documentation if needed
• My branch is up-to-date with dev