fix(gui): correct relative import that broke app startup — 0.18.2

The recording indicator (0.18.0) used `from .core import diagnostic`, which
resolves to the non-existent rigdoctor.gui.core and crashed MainWindow on launch.
Fixed to `from ..core import diagnostic`.

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 = "0.18.1"
+version = "0.18.2"
 description = "Modular hardware monitoring & crash diagnostics for Linux gamers."
 readme = "README.md"
 requires-python = ">=3.11"