Add a `Cancel` button to long-running scans (Duplicates, Smart Scan)

[iliyami]

Right now if a user starts the Duplicates scan and it's taking too long, they have to force-quit. We should let them cancel cleanly.

Task

1. In ScanCoordinator.swift, the cancel() method exists but isn't exposed in views.
2. Add a small "Cancel" button next to the ScanProgressRing in:
   • SmartScanView.swift (scanning state)
   • DuplicatesView.swift (scanning state)
3. The button should call appState.scanCoordinator.cancel() and transition back to .idle.

Acceptance criteria

• Clicking Cancel during a scan returns to the idle view within 1 second
• No crashes or orphan tasks

Files to change

• Sources/MacClean/Views/SmartScan/SmartScanView.swift
• Sources/MacClean/Views/Files/DuplicatesView.swift