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/rendering/renderer.cpp

@@ -1,7 +1,6 @@
 #include "rendering/renderer.hpp"
 #include "rendering/camera.hpp"
 #include "rendering/camera_controller.hpp"
-#include "rendering/scene.hpp"
 #include "rendering/terrain_renderer.hpp"
 #include "rendering/terrain_manager.hpp"
 #include "rendering/performance_hud.hpp"
@@ -26,7 +25,6 @@
 #include "rendering/minimap.hpp"
 #include "rendering/world_map.hpp"
 #include "rendering/quest_marker_renderer.hpp"
-#include "rendering/shader.hpp"
 #include "game/game_handler.hpp"
 #include "pipeline/m2_loader.hpp"
 #include <algorithm>
@@ -673,9 +671,6 @@ bool Renderer::initialize(core::Window* win) {
     cameraController->setUseWoWSpeed(true);  // Use realistic WoW movement speed
     cameraController->setMouseSensitivity(0.15f);
 
-    // Create scene
-    scene = std::make_unique<Scene>();
-
     // Create performance HUD
     performanceHUD = std::make_unique<PerformanceHUD>();
     performanceHUD->setPosition(PerformanceHUD::Position::TOP_LEFT);
@@ -877,7 +872,6 @@ void Renderer::shutdown() {
     zoneManager.reset();
 
     performanceHUD.reset();
-    scene.reset();
     cameraController.reset();
     camera.reset();