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/core/UsdSceneRenderer.h

@@ -31,6 +31,15 @@ enum class BBoxMode {
     AllSelection    ///< One combined box for the entire selection
 };
 
+/// Surface shading mode — maps to UsdImagingGLDrawMode + enableLighting.
+enum class ShadingMode {
+    SmoothShaded,        ///< Smooth shading with materials (default)
+    FlatShaded,          ///< Flat shading with materials
+    WireframeOnSurface,  ///< Wireframe overlay on solid surface
+    Wireframe,           ///< Wireframe only, no fill
+    Unlit,               ///< Smooth geometry, lighting disabled
+};
+
 class UsdSceneRenderer {
 public:
     UsdSceneRenderer();
@@ -199,6 +208,9 @@ public:
     bool GetDomeLightEnabled()              const { return m_domeLightEnabled; }
     void SetDomeLightEnabled(bool val)            { m_domeLightEnabled = val; }
 
+    ShadingMode GetShadingMode()             const { return m_shadingMode; }
+    void        SetShadingMode(ShadingMode m)     { m_shadingMode = m; m_forceRefresh = true; }
+
     /// Default material ambient (kA, default 0.2 — matches viewSettingsDataModel.py).
     float GetDefaultMaterialAmbient()       const { return m_defaultMaterialAmbient; }
     void  SetDefaultMaterialAmbient(float v)      { m_defaultMaterialAmbient = v; }
@@ -298,6 +310,7 @@ private:
     pxr::GfVec3f    m_backgroundColor;
 
     // Lighting settings (mirrors stageView.py viewSettings)
+    ShadingMode     m_shadingMode;            // draw mode + lighting flags
     bool            m_ambientLightOnly;       // camera headlight
     bool            m_domeLightEnabled;       // dome/IBL light
     bool            m_stageIsZup;             // used for dome light rotation