Fix camera orbit rotation - use camera.up and horizontal orbit

Fixes:
1. Camera now orbits horizontally (not vertically)
2. Uses camera.up instead of hardcoded global up vector

Technical changes:
- Remove hardcoded global up vector
- Use render_state->camera.up for rotation axis
- Rotate forward vector around camera's up vector
- This creates proper horizontal orbit around target

Rotation formula:
new_forward = forward * cos(angle) + cross(up, forward) * sin(angle)

Result: Camera orbits horizontally around target at any
camera position, using the camera's own up vector
for proper orientation-aware rotation.

src/renderer.cpp

@@ -59,23 +59,21 @@ void update_camera(RenderState* render_state, SimulationState* sim) {
         render_state->camera.position = Vector3Add(body_pos, render_state->camera_offset);
     }
 
-    // Camera rotation with proper up vector
-    Vector3 up = (Vector3){ 0.0f, 1.0f, 0.0f };  // Global up for space
+    // Camera rotation using camera's up vector
     Vector3 to_camera = Vector3Subtract(render_state->camera.position, render_state->camera.target);
     float camera_distance = Vector3Length(to_camera);
 
-    // Rotate around target using cross products
+    // Rotate around target using camera's up vector
     if (IsKeyDown(KEY_LEFT)) {
-        Vector3 right = Vector3Normalize(Vector3CrossProduct(up, to_camera));
         Vector3 forward = Vector3Normalize(to_camera);
 
-        // Rotate forward vector around right axis
+        // Rotate forward vector around camera's up axis (horizontal orbit)
         float cos_a = cosf(angle_speed);
         float sin_a = sinf(angle_speed);
 
         Vector3 new_forward = Vector3Add(
             Vector3Scale(forward, cos_a),
-            Vector3Scale(Vector3CrossProduct(right, forward), sin_a)
+            Vector3Scale(Vector3CrossProduct(render_state->camera.up, forward), sin_a)
         );
 
         render_state->camera.position = Vector3Add(
@@ -91,16 +89,15 @@ void update_camera(RenderState* render_state, SimulationState* sim) {
         }
     }
     if (IsKeyDown(KEY_RIGHT)) {
-        Vector3 right = Vector3Normalize(Vector3CrossProduct(up, to_camera));
         Vector3 forward = Vector3Normalize(to_camera);
 
-        // Rotate forward vector around right axis
+        // Rotate forward vector around camera's up axis (horizontal orbit)
         float cos_a = cosf(-angle_speed);
         float sin_a = sinf(-angle_speed);
 
         Vector3 new_forward = Vector3Add(
             Vector3Scale(forward, cos_a),
-            Vector3Scale(Vector3CrossProduct(right, forward), sin_a)
+            Vector3Scale(Vector3CrossProduct(render_state->camera.up, forward), sin_a)
         );
 
         render_state->camera.position = Vector3Add(