feat(dashboard): add lock button to prevent accidental widget movement

- Add lock/unlock button to dashboard header (always visible)
- Lock state prevents dragging in both normal and edit modes
- Lock state prevents resizing in edit mode
- Icon changes: lock-open (unlocked) ↔ lock (locked)
- Hide resize handles and prevent grab cursor when locked
- Lock state persists across edit mode toggles
- Integrate lock checks in DragDropHandler and ResizeHandler
- Pass editManager reference to drag/resize handlers for lock state access

src/systems/dashboard/dashboardManager.js

@@ -156,21 +156,7 @@ export class DashboardManager {
             }
         });
 
-        // Initialize Drag & Drop
-        this.dragHandler = new DragDropHandler(this.gridEngine, {
-            showGrid: true,
-            enableSnap: true
-        });
-
-        // Initialize Resize Handler
-        this.resizeHandler = new ResizeHandler(this.gridEngine, {
-            minWidth: 1,
-            minHeight: 2,
-            maxWidth: 4, // Max 4 columns (will be clamped to actual column count)
-            maxHeight: 10
-        });
-
-        // Initialize Edit Mode Manager
+        // Initialize Edit Mode Manager first (needed by drag/resize handlers)
         this.editManager = new EditModeManager({
             container: this.container,
             onSave: () => this.handleEditSave(),
@@ -180,6 +166,22 @@ export class DashboardManager {
             onWidgetSettings: (widgetId) => this.openWidgetSettings(widgetId)
         });
 
+        // Initialize Drag & Drop (with editManager reference)
+        this.dragHandler = new DragDropHandler(this.gridEngine, {
+            showGrid: true,
+            enableSnap: true,
+            editManager: this.editManager
+        });
+
+        // Initialize Resize Handler (with editManager reference)
+        this.resizeHandler = new ResizeHandler(this.gridEngine, {
+            minWidth: 1,
+            minHeight: 2,
+            maxWidth: 4, // Max 4 columns (will be clamped to actual column count)
+            maxHeight: 10,
+            editManager: this.editManager
+        });
+
         // Initialize Layout Persistence
         this.persistence = new LayoutPersistence({
             debounceMs: this.config.debounceMs,