refactor: complete OpenGL→Vulkan migration (Phase 7)

Remove all OpenGL/GLEW code and dependencies. The Vulkan renderer has
been the sole active backend for months; these files were dead code.

Deleted (8 files, 641 lines):
- rendering/mesh.cpp+hpp: OpenGL VAO/VBO/EBO wrapper (never instantiated)
- rendering/shader.cpp+hpp: OpenGL GLSL compiler (replaced by VkShaderModule)
- rendering/scene.cpp+hpp: Scene graph holding Mesh objects (created but
  never populated — all rendering uses Vulkan sub-renderers directly)
- rendering/video_player.cpp+hpp: FFmpeg+GL texture uploader (never
  included by any other file — login video feature can be re-implemented
  with VkTexture when needed)

Cleaned up:
- renderer.hpp: remove Scene forward-decl, getScene() accessor, scene member
- renderer.cpp: remove scene.hpp/shader.hpp includes, Scene create/destroy
- application.cpp: remove stale "GL/glew.h removed" comment
- CMakeLists.txt: remove find_package(OpenGL/GLEW), source/header entries,
  and target_link_libraries for OpenGL::GL and GLEW::GLEW
- PKGBUILD: remove glew dependency
- BUILD_INSTRUCTIONS.md: remove glew from all platform install commands

src/core/application.cpp

@@ -48,7 +48,6 @@
 #include "pipeline/dbc_layout.hpp"
 
 #include <SDL2/SDL.h>
-// GL/glew.h removed — Vulkan migration Phase 1
 #include <cstdlib>
 #include <climits>
 #include <algorithm>