Add transition support for Sonoff MINI-DIM

[azgooon]

Hey! I've been using the MINI-DIM (UIID 277) with an adaptive lighting automation and noticed it was the only dimmer class without transition support. Thought I'd have a go at adding it.

What I found

The MINI-DIM firmware (tested on 1.1.2) supports a transitionTime parameter in milliseconds — the device does smooth hardware fading when it receives it alongside a brightness command. The catch is it only works through the cloud API. The local /zeroconf endpoint either ignores it or returns a text/html error and times out.

What this does

• Adds LightEntityFeature.TRANSITION to XMiniDim
• When transition is passed to light.turn_on, the brightness is sent via local as usual (fast), then the full command including transitionTime is fired through cloud in the background using asyncio.create_task so the caller isn't blocked
• When no transition is requested, behaviour is exactly the same as before

Testing

Tested on my MINI-DIM (firmware 1.1.2) with:

• Manual light.turn_on calls with transition: 3 and transition: 5 — smooth fades both up and down
• HA scenes with transitions
• Automation loops (works but cloud latency means transitions arrive slightly after the local command)
• No transition (existing behaviour) — unchanged

Happy to adjust if there's a better way to handle the local/cloud split. Cheers!