frontend: Waypoints numbers not sequential

### Current behaviour

When planning a mission with command waypoints, If the user adds a command, the wp numbers stops being sequential.

E.g. if you add a command on wp2, wp3 vanishes and wp4 takes its place (image below)

### Expected or desired behaviour

All wp should be sequentially numbered

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.

Reproduce the issue in the frontend mission-planning flow by adding a command waypoint at wp2 and observing the displayed numbers. Trace the waypoint-numbering entry point and verify that all displayed waypoints remain sequential after insertion, including the reported wp2/wp3/wp4 case.

Written by the indexing model from the issue text.