Add gear icon toolbar button with viewport options dropdown

Adds a ⚙ icon button to the right of the AA toggle in each viewport
tile's compact toolbar.  Clicking it opens a dropdown with:
  - Shading submenu: Smooth Shaded / Flat Shaded / Wireframe on Surface /
    Wireframe / Unlit  (new ShadingMode enum driving UsdImagingGLDrawMode)
  - Lighting submenu: Camera Headlight / Scene Lights + Dome Light toggle
  - Show Grid and Anti-Aliasing toggles (mirrored from icon buttons)
  - Bounding Box submenu: None / Per Object / All Selection
  - Background submenu: Dark Gray / Black / Light Gray / Midnight Blue

ShadingMode is persisted in viewport_settings.ini alongside the existing
per-tile settings (saved on shutdown, restored on startup).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/ViewportTile.h

@@ -28,6 +28,7 @@ struct ViewportTileSettings {
     int         bboxMode         = 0;     ///< BBoxMode cast to int
     bool        ambientLightOnly = false;
     bool        domeLightEnabled = false;
+    int         shadingMode      = 0;     ///< ShadingMode cast to int (0 = SmoothShaded)
 };
 
 /// Named orthographic view directions.