feat: add window resize support

Added width/height tracking to the window class via a GLFW
resize callback. The callback stores the current size and
forwards it to a user-provided std::function.

Added context::set_size() to update the GL viewport when
the window is resized.

Changed scene::render() to accept (int width, int height)
parameters so scenes can compute the correct aspect ratio
for their projection matrix instead of hardcoding 1280/720.

Fixed parameter shadowing in resize_callback_impl
(glfw_window instead of window).

cbt/opengl/context.hpp

@@ -10,6 +10,7 @@ public:
     ~context();
 
     auto valid() const -> bool;
+    auto set_size(int width, int height) -> void;
 
 private:
     bool m_valid = false;