feat(editor): center on terrain (Home key), navigation improvements

- "Center on Terrain" (Home key or View menu): resets camera to center
  of loaded tile at 300 units altitude with 45-degree downward pitch.
  Essential for recovering when camera gets lost in empty space.
- Toast confirmation on center action

tools/editor/editor_app.cpp

@@ -206,6 +206,7 @@ void EditorApp::processEvents() {
                 auto sc = event.key.keysym.scancode;
                 if (sc == SDL_SCANCODE_F3) setWireframe(!isWireframe());
                 if (sc == SDL_SCANCODE_F5) saveBookmark("");
+                if (sc == SDL_SCANCODE_HOME) centerOnTerrain();
                 // Number keys switch modes (when not typing in ImGui)
                 if (!io.WantCaptureKeyboard) {
                     if (sc == SDL_SCANCODE_1) setMode(EditorMode::Sculpt);
@@ -788,6 +789,15 @@ void EditorApp::flyToSelected() {
     }
 }
 
+void EditorApp::centerOnTerrain() {
+    if (!terrain_.isLoaded()) return;
+    float centerX = (32.0f - loadedTileY_) * 533.33333f - 8.0f * 533.33333f / 16.0f;
+    float centerY = (32.0f - loadedTileX_) * 533.33333f - 8.0f * 533.33333f / 16.0f;
+    camera_.setPosition(glm::vec3(centerX, centerY, terrain_.chunks[0].position[2] + 300.0f));
+    camera_.setYawPitch(0.0f, -45.0f);
+    showToast("Camera centered on terrain");
+}
+
 void EditorApp::snapSelectedToGround() {
     auto* sel = objectPlacer_.getSelected();
     if (!sel || !terrain_.isLoaded()) return;

tools/editor/editor_app.hpp

@@ -73,6 +73,7 @@ public:
     void setSkyPreset(int preset); // 0=day, 1=dusk, 2=night
     void snapSelectedToGround();
     void flyToSelected();
+    void centerOnTerrain();
 
     // Multi-tile support
     void addAdjacentTile(int offsetX, int offsetY);

tools/editor/editor_ui.cpp

@@ -148,6 +148,7 @@ void EditorUI::renderMenuBar(EditorApp& app) {
             bool wf = app.isWireframe();
             if (ImGui::MenuItem("Wireframe", "F3", &wf)) app.setWireframe(wf);
             if (ImGui::MenuItem("Reset Camera")) app.resetCamera();
+            if (ImGui::MenuItem("Center on Terrain", "Home")) app.centerOnTerrain();
             ImGui::Separator();
             if (ImGui::BeginMenu("Sky / Lighting")) {
                 if (ImGui::MenuItem("Day")) app.setSkyPreset(0);