Add UI screenshots and pyright config

Add Screenshots/ (panel + settings tabs: engines, input, general, benchmark,
about) for documentation, and pyrightconfig.json pointing the type checker at
the system dist-packages so it resolves PyGObject (gi).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pyrightconfig.json

@@ -0,0 +1,5 @@
+{
+  "extraPaths": [
+    "/usr/lib/python3/dist-packages"
+  ]
+}