Add Noir Gear M1-NEX artwork mapping

[williamsnoirgear]

Summary

• Map the reported Noir Gear M1-NEX model name to the reserved noir-m1-nex.png device artwork filename.
• Add regression coverage for both M1-NEX name forms and ensure Noir M1 / M1 Lite do not match.

Related to the separate artwork request: #678. The source asset is provided there; this PR does not add a binary. Per public/devices/README.md, a maintainer uploads and normalizes the image in the device bucket. Until that happens, the current missing-art behavior remains graceful. This mapping is independent of the pending wired protocol support in OpenMouse-Project/mouse-protocol#182.

Validation

• TypeScript tsc --noEmit — passed
• Vite production build — passed (existing chunk-size and use client warnings)
• OpenMouse test suite — 293 passed, 0 failed