Skylapse 0.5.5 — test the heater before trusting it

Adds a five-second heater test to the dew heater card, for the moment you first power one on: it drives the heater briefly so you can check the wiring and feel for warmth, without waiting for the weather to reach the dewpoint.

Capped at fifteen seconds however it is called, and the heater is switched off even if the test fails or the page disconnects part-way through. A heater left on unattended is the one genuinely dangerous thing this hardware can do, so the tool for testing it cannot be the thing that causes it.