Add desktop GUI (M10): modern dark dashboard

PySide6/Qt front-end over the stdlib sensor core (only gui/ imports Qt).
- sidebar navigation + stacked pages (Dashboard live; Logs/Health/Inventory
  placeholders for M3-M5)
- live dashboard: circular gauges (GPU temp/load, CPU temp, memory) plus
  collapsible per-subsystem cards with progress bars and metric rows
- background sampling thread -> Qt signal so the UI stays responsive
- temperature colors: icey-blue (cold) -> green -> orange -> red (hot)
- dark theme via QSS + Fusion

Supporting changes:
- cpu source: order temps as package, then cores numerically (fixes CLI too)
- render: expose format_value/metric_label, shared by CLI and GUI
- cli: `rigdoctor gui` (lazy import; prints install hint if PySide6 missing)
- pyproject: rigdoctor-gui script + [gui] extra (PySide6)
- gitignore: *.egg-info/, build/, dist/

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

pyproject.toml

@@ -17,6 +17,7 @@ gui = ["PySide6"]
 
 [project.scripts]
 rigdoctor = "rigdoctor.cli:main"
+rigdoctor-gui = "rigdoctor.gui.app:main"
 
 [tool.setuptools.packages.find]
 where = ["src"]