Add a custom ring colour scheme

[PyFlat-JR]

What does this PR do?

Adds a custom ring colour scheme, for example for a black and white deck.

• Custom colour in the Colours list, with Ring colour (default white) for every ring, icon, bolt and list bar.
• Low battery in red (default on) keeps a level at or below the threshold red. Off, low devices use the ring colour too. Every other scheme still always shows low in red.
• A stale or unknown reading stays grey either way.
• A ring colour that isn't #rrggbb falls back to white, since shapes take a hex only.
• New previews: "Panel - black and white", "Tile - black and white, low not red".

Checklist

• I have read AGENTS.md and CONTRIBUTING.md.
• dotnet build and dotnet test pass locally (0 warnings, 275 tests).
• I updated README.md / AGENTS.md if this changes what they describe.
• If this touches a device backend: I tested it against the real hardware (see below).

Device/hardware tested against (if applicable)

Not a device backend.