New Engine/Core/Input.h/.cpp: static input facade with a Key/MouseButton
enum (values map to GLFW key codes internally) so scripts and game logic
no longer touch GLFW directly - window backend becomes swappable and the
facade is the surface to expose when embedding scripting later. Wired up
in Application's constructor; Camera (editor camera WASD/RMB/MMB), the
WASDMove script and SandboxApp key handling now query Input only.
Ctrl+P hotkey fix: the !m_Playing guard made the hotkey dead exactly
when it was needed - after pressing Play. Now toggles both ways.
Toolbar Pause button now reads "Resume" while paused (tooltip follows).