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>

resources/icons/gear.svg

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