fix(gui): high-contrast checkbox indicator

A checked checkbox was hard to distinguish from unchecked on the dark theme.
Style QCheckBox::indicator: accent-filled box with a checkmark when checked,
dark box with a visible border when not.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,11 @@ All notable changes to RigDoctor are recorded here. Format follows
 (`MAJOR.MINOR.PATCH`, pre-1.0). `__version__` and `pyproject.toml` must match the git
 release tag (so the auto-updater, D18, can compare versions).
 
+## [0.4.1] - 2026-05-21
+### Fixed
+- Checkbox contrast: a checked checkbox is now a clear accent-filled box with a checkmark
+  (was hard to tell checked from unchecked on the dark theme).
+
 ## [0.4.0] - 2026-05-21
 ### Added
 - **Alerts (M8)**: desktop notifications (via `notify-send`) for **overheat** (GPU/CPU past a