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

include/rendering/renderer.hpp

@@ -30,7 +30,6 @@ namespace rendering {
 
 class Camera;
 class CameraController;
-class Scene;
 class TerrainRenderer;
 class TerrainManager;
 class PerformanceHUD;
@@ -54,7 +53,6 @@ class Minimap;
 class WorldMap;
 class QuestMarkerRenderer;
 class CharacterPreview;
-class Shader;
 class AmdFsr3Runtime;
 
 class Renderer {
@@ -119,7 +117,6 @@ public:
 
     Camera* getCamera() { return camera.get(); }
     CameraController* getCameraController() { return cameraController.get(); }
-    Scene* getScene() { return scene.get(); }
     TerrainRenderer* getTerrainRenderer() const { return terrainRenderer.get(); }
     TerrainManager* getTerrainManager() const { return terrainManager.get(); }
     PerformanceHUD* getPerformanceHUD() { return performanceHUD.get(); }
@@ -219,7 +216,6 @@ private:
     core::Window* window = nullptr;
     std::unique_ptr<Camera> camera;
     std::unique_ptr<CameraController> cameraController;
-    std::unique_ptr<Scene> scene;
     std::unique_ptr<TerrainRenderer> terrainRenderer;
     std::unique_ptr<TerrainManager> terrainManager;
     std::unique_ptr<PerformanceHUD> performanceHUD;