feat(editor): status bar shows active biome

Adds a public EditorApp::getActiveBiome() accessor and renders the
biome name in the status bar between the tile coords and the
dirty marker. So the bar reads, e.g., "[Sculpt] mymap [32,32]
Forest *" — at a glance the user knows what biome the painting/
auto-paint passes will use.

Useful especially after creating a new terrain: the biome
selection previously only mattered at create time + during
"Generate Complete Zone", with no visible feedback that it was
remembered.

tools/editor/editor_app.hpp

@@ -207,6 +207,9 @@ private:
     // reads this so subsequent generations honor the active biome instead of
     // reapplying the same hardcoded heightband textures every time.
     Biome activeBiome_ = Biome::Grassland;
+public:
+    Biome getActiveBiome() const { return activeBiome_; }
+private:
 
     // "Click on terrain to place crater" mode. The Crater button arms this
     // with the user's chosen radius/depth/rim; the next left-click on