Prepare v4.2.0 release

[PurpleSentinel]

Summary

• set firmware and system-default version metadata to 4.2.0
• promote the automatic-orientation and smooth-gradient work into the v4.2 changelog
• mark v4.2.0 as current across the README and User Guide

Why

The default branch contains two completed feature sets after v4.1.0: continuous automatic display orientation and the proportional timer colour gradient. A new minor release records that tested state and keeps the on-device version, documentation, tag, and GitHub release aligned.

Impact

• no runtime behaviour changes beyond the already merged and hardware-validated features
• existing user configuration remains compatible
• startup and hardware-information screens report v4.2.0 after deployment

Validation

• python -m unittest discover -s tests — 143 tests passed
• issue Add continuous automatic display orientation #64 Auto rotation was hardware-validated on the supported RP2040
• issue Add a smooth progress-based timer colour gradient #66 gradient and deep-purple overrun were hardware-validated on the supported RP2040
• git diff --cached --check passed