feat(editor): heightmap import, toast notifications, workflow polish

- Import Heightmap: File > Import Heightmap loads RAW 8/16-bit grayscale
  files (129x129 or 257x257) and maps to terrain heights with configurable
  scale. Supports standard terrain editor heightmap formats.
- Toast notifications: non-intrusive green popup at bottom center for
  user feedback (save confirmations, import results, errors)
- Toasts fade out after 3 seconds with alpha animation
- Auto-save now shows toast on save
- Quick-save (Ctrl+S) shows toast confirmation

tools/editor/terrain_editor.hpp

@@ -54,6 +54,9 @@ public:
     // Noise generator: applies procedural height noise to the terrain
     void applyNoise(float frequency, float amplitude, int octaves, uint32_t seed);
 
+    // Import heightmap from raw 16-bit grayscale (129x129 or 257x257)
+    bool importHeightmap(const std::string& path, float heightScale);
+
     // Water editing
     void setWaterLevel(const glm::vec3& center, float radius, float waterHeight, uint16_t liquidType = 0);
     void removeWater(const glm::vec3& center, float radius);