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>

CHANGELOG.md

@@ -5,6 +5,12 @@ 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).
 
+## [0.18.2] - 2026-05-22
+### Fixed
+- **GUI wouldn't start** (0.18.0 regression): the recording indicator used a wrong relative
+  import (`from .core` → `rigdoctor.gui.core`, which doesn't exist), crashing `MainWindow` on
+  launch. Corrected to `from ..core`.
+
 ## [0.18.1] - 2026-05-22
 ### Changed
 - Recording badge: dropped the sample count (not useful at a glance) — it now shows just