Fix: raise Disk Health bay label contrast

[SandObserver]

Summary

• Raise Disk Health bay names to 62% white (5.48:1 on the bay, 4.65:1 on a pressed bay).
• Raise unread bay names and bay numbers to 50% white (4.72:1).
• Add a test that computes each text and fill pair from the widget's CSS and requires 4.5:1.

Testing
All green, e2e included. Added 4 contrast tests.

Changelog

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.