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/window.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <functional>
 #include <string>
 
 #define GLFW_INCLUDE_NONE
@@ -19,13 +20,22 @@ public:
     auto raw() const -> GLFWwindow*;
     auto stop() -> void;
     auto screenshot(std::string filepath = "screenshot.png") const -> bool;
+    auto width() const -> int;
+    auto height() const -> int;
+
+    using resize_callback = std::function<void(int width, int height)>;
+    auto on_resize(resize_callback cb) -> void;
 
 private:
     GLFWwindow* m_window = nullptr;
+    int m_width = 0;
+    int m_height = 0;
+    resize_callback m_resize_cb;
     bool m_glfw_initialized = false;
 
     static auto init_glfw() -> bool;
     static auto terminate_glfw() -> void;
+    static auto resize_callback_impl(GLFWwindow* glfw_window, int width, int height) -> void;
 };
 
 }