Preserve camera distance when switching bodies

Changes:
- Add previous_selected_body to RenderState
- Detect when selected_body_index changes
- Recalculate camera_offset when switching to new body
- Maintains same camera distance/orientation to new body

Technical details:
- Body selection change detected via previous_selected_body comparison
- When switching to new body:
  * Calculate new body position
  * Set camera target to new body
  * Use existing camera_offset (maintains distance)
  * Update camera.position = body_pos + offset
- Allows seamless transitions between bodies at same zoom level

Result: Camera maintains perspective when switching between
celestial bodies, no jarring distance changes.

src/main.cpp

@@ -38,6 +38,7 @@ void run_gui_simulation(SimulationState* sim) {
 
     // Initialize UI state
     render_state.selected_body_index = -1;  // No selection initially
+    render_state.previous_selected_body = -1;  // Previous selected body index
     render_state.body_list_scroll = 0;     // Initial scroll position
     render_state.body_list_active = -1;    // No active item initially
     render_state.camera_follow_body = false; // Camera not following initially

src/renderer.cpp

@@ -49,6 +49,17 @@ void update_camera(RenderState* render_state, SimulationState* sim) {
         }
     }
 
+    // Preserve distance when switching to different body
+    if (render_state->camera_follow_body && render_state->selected_body_index >= 0 &&
+        render_state->selected_body_index != render_state->previous_selected_body &&
+        render_state->selected_body_index < sim->body_count) {
+        // Body selection changed - recalculate offset to maintain distance
+        Vector3 body_pos = sim_to_render(sim->bodies[render_state->selected_body_index].position, render_state->distance_scale);
+        render_state->camera.target = body_pos;
+        // Use current offset to maintain distance to new body
+        render_state->camera.position = Vector3Add(body_pos, render_state->camera_offset);
+    }
+
     // Update target position when following
     if (render_state->camera_follow_body && render_state->selected_body_index >= 0 &&
         render_state->selected_body_index < sim->body_count) {
@@ -136,8 +147,9 @@ void update_camera(RenderState* render_state, SimulationState* sim) {
         }
     }
 
-    // Store previous follow state
+    // Store previous follow state and selected body
     render_state->was_following_body = render_state->camera_follow_body;
+    render_state->previous_selected_body = render_state->selected_body_index;
 }
 
 // Transform from simulation coordinates (XY plane) to render coordinates (XZ plane)

src/renderer.h

@@ -10,6 +10,7 @@ struct RenderState {
     double distance_scale;    // Scale factor for distances
     double size_scale;        // Scale factor for body sizes
     int selected_body_index;  // -1 = no selection
+    int previous_selected_body;  // Previous selected body index
     int body_list_scroll;     // Scroll position for body list
     int body_list_active;     // Active item index in body list
     bool camera_follow_body;  // Whether camera follows selected body