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

style.css

@@ -1172,6 +1172,17 @@ body:has(.rpg-panel.rpg-position-left) #sheld {
     pointer-events: auto;
 }
 
+/* Hide resize handles when widgets are locked */
+.widgets-locked .resize-handles {
+    opacity: 0 !important;
+    pointer-events: none !important;
+}
+
+/* Prevent grab cursor when widgets are locked */
+.widgets-locked .rpg-widget {
+    cursor: default !important;
+}
+
 /* ========================================
    DASHBOARD V2 WIDGET STYLES
    ========================================