Add global Preferences dialog and per-viewport render delegate

Preferences (Edit > Preferences, tabbed General/Animation/Viewport):
- Viewport tab: Default Render Delegate and Color Correction settings
- Persisted to %APPDATA%\UsdLayerManager\preferences.ini
- Render delegate: startup default only (not live-applied to running tiles)
- Color correction: global, applied live to all viewports on change

Render delegate per-viewport:
- Each tile toolbar switches its own delegate independently
- Saved per-tile in viewport_settings.ini; restored on next launch
- Tiles with no saved delegate get the pref default on startup

Color correction global:
- Removed from gear popup; lives in Preferences > Viewport only
- Applied to all tiles on init and on live change from Preferences

Fix InitRenderer always applying m_currentRendererPlugin when set,
rather than skipping when UsdImagingGLEngine auto-selects a default.

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

src/ui/ViewportPanel.h

@@ -52,7 +52,7 @@ public:
     // ── Main render (called from Application::RenderUI) ──────────────────────
     void Render(bool* p_open = nullptr);
 
-    // ── Pick callbacks (wired by Application after construction) ─────────────
+    // ── Callbacks (wired by Application after construction) ──────────────────
     std::function<void(const std::string&)>              OnPrimPicked;
     std::function<void(const std::vector<std::string>&)> OnPrimsPickedRect;
 
@@ -65,9 +65,18 @@ public:
     LayoutMode GetLayout()           const { return m_layout;           }
     int        GetFocusedTileIndex() const { return m_focusedTileIndex; }
 
+    // ── Global preferences ────────────────────────────────────────────────────
+    /// Set render delegate on tiles that have no saved delegate yet (empty token).
+    /// Called once on startup so the pref default applies to fresh tiles.
+    void ApplyDefaultDelegate(const std::string& delegate);
+    /// Apply color correction to every tile (always global).
+    void ApplyGlobalColorCorrection(int ccMode, const std::string& ocioDisplay,
+                                    const std::string& ocioView,
+                                    const std::string& ocioColorSpace,
+                                    const std::string& ocioLook);
+
     // ── Settings persistence ─────────────────────────────────────────────────
     /// Load layout and per-tile render settings from a simple INI file.
-    /// Safe to call immediately after construction (before any Render call).
     void LoadSettings(const std::string& path);
     /// Save current layout and per-tile render settings to a simple INI file.
     void SaveSettings(const std::string& path) const;