refactor(gameplay): extract Entity base class from ClientPlayer

Move position, walk pose, velocity, orientation, movement, and gravity
fields and their accessors into a new Entity base class. ClientPlayer now
inherits from Entity, removing duplicated members. Also update velocity
handling to use 3D vector per axis and adjust related logic.

src/CMakeLists.txt

@@ -97,4 +97,5 @@ target_sources(${PROJECT_NAME}
         render/model_renderer.cpp
         gameplay/chunk.cpp
         gameplay/hitbox_manager.cpp
+        gameplay/entity.cpp
 )