Add image editor with annotation tools

Integrates an editor accessible via "Letzte Aufnahme bearbeiten…" or
"Bild bearbeiten…" in the menu bar. Supports arrow, rectangle, ellipse,
and text annotations with color picker, line width, undo/redo, and
export to file or clipboard.

New files in Sources/Editor/:
- AnnotationItem/Document: data model and state management
- AnnotationCanvasView: NSView with CG rendering and mouse interaction
- AnnotationRenderer: shared rendering for canvas and image export
- EditorToolbarView: SwiftUI toolbar with tool selection and actions
- EditorWindowController: window lifecycle following SettingsWindowController pattern

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Sources/SettingsWindowController.swift

@@ -32,7 +32,9 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
     }
 
     func windowWillClose(_ notification: Notification) {
-        // Menu bar-only apps should not keep a Dock icon after the window closes.
-        NSApplication.shared.setActivationPolicy(.accessory)
+        let otherVisible = NSApplication.shared.windows.filter { $0.isVisible && $0 != window }
+        if otherVisible.isEmpty {
+            NSApplication.shared.setActivationPolicy(.accessory)
+        }
     }
 }