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/Editor/AnnotationDocument.swift

@@ -0,0 +1,56 @@
+import AppKit
+import Foundation
+
+@Observable
+final class AnnotationDocument {
+    var items: [AnnotationItem] = []
+    var selectedItemID: UUID?
+    var currentTool: AnnotationKind = .arrow
+    var currentColor: NSColor = .systemRed
+    var currentLineWidth: CGFloat = 3
+
+    private var undoStack: [[AnnotationItem]] = []
+    private var redoStack: [[AnnotationItem]] = []
+
+    var canUndo: Bool { !undoStack.isEmpty }
+    var canRedo: Bool { !redoStack.isEmpty }
+
+    func pushUndo() {
+        undoStack.append(items)
+        redoStack.removeAll()
+    }
+
+    func undo() {
+        guard let previous = undoStack.popLast() else { return }
+        redoStack.append(items)
+        items = previous
+        selectedItemID = nil
+    }
+
+    func redo() {
+        guard let next = redoStack.popLast() else { return }
+        undoStack.append(items)
+        items = next
+        selectedItemID = nil
+    }
+
+    func addItem(_ item: AnnotationItem) {
+        pushUndo()
+        items.append(item)
+        selectedItemID = item.id
+    }
+
+    func deleteSelected() {
+        guard let id = selectedItemID else { return }
+        pushUndo()
+        items.removeAll { $0.id == id }
+        selectedItemID = nil
+    }
+
+    func moveSelected(by delta: CGSize) {
+        guard let id = selectedItemID,
+              let index = items.firstIndex(where: { $0.id == id }) else { return }
+        items[index].rect.origin.x += delta.width
+        items[index].rect.origin.y += delta.height
+    }
+}