Correct POI orbit height range

Fixes POI default orbit heights so the three automatic rings are distributed between the configured Minimum flight height and Flight height instead of adding +20 m / +40 m above the configured flight height.
Example: Minimum 50 m and Flight height 100 m now creates 50 / 75 / 100 m POI rings.
Repairs legacy 0.4.2.6 POI defaults such as 120 / 140 / 160 m automatically when the mission is opened, including browser drafts and stored deterministic POI routes.
Keeps manually edited POI orbit heights inside the configured mission height range and updates automatic orbit heights live when Minimum flight height or Flight height changes.
Prevents the optional 3D perimeter-ring helper from introducing an altitude above 120 m when the base 3D mapping altitude itself is at or below 120 m.
Keeps the existing above-120 m warning for intentionally configured operations above 120 m; the warning is no longer triggered by the faulty POI defaults.