feat: edge edit mode + proxmox container mode toggle

- Double-click any link to open Edit Link modal (type, label, VLAN ID, delete)
- Add updateEdge / deleteEdge actions to canvasStore
- Add container_mode field to proxmox nodes (backend model, schemas, migration)
- NodeModal shows container toggle for proxmox type (defaults ON)
- ProxmoxGroupNode renders as regular BaseNode when container_mode is OFF
- setProxmoxContainerMode store action handles structural changes atomically
  (children parentId/extent, node dimensions)
- CanvasContainer filters edges between container proxmox and its children
- Canvas load respects container_mode when assigning parentId/extent

backend/app/schemas/nodes.py

@@ -19,6 +19,7 @@ class NodeBase(BaseModel):
     pos_x: float = 0
     pos_y: float = 0
     parent_id: str | None = None
+    container_mode: bool = False
 
 
 class NodeCreate(NodeBase):
@@ -39,6 +40,7 @@ class NodeUpdate(BaseModel):
     notes: str | None = None
     pos_x: float | None = None
     pos_y: float | None = None
+    container_mode: bool | None = None
 
 
 class NodeResponse(NodeBase):