Add keyboard shortcuts for Smart Scan and Clean (⌘R, ⌘K)

Power users expect keyboard shortcuts.

### Task
- ⌘R: Start a new scan in the current view
- ⌘K: Clean (when results are showing)
- ⌘1-⌘9: Jump to sidebar items

Use `.keyboardShortcut()` modifier in SwiftUI.

### Files to change
- `Sources/MacClean/App/ContentView.swift`
- Module views