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>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "rigdoctor"
-version = "0.4.0"
+version = "0.4.1"
 description = "Modular hardware monitoring & crash diagnostics for Linux gamers."
 readme = "README.md"
 requires-python = ">=3.11"