Port UI icon textures from OpenGL to Vulkan, fix loading screen clear values

Replace all glGenTextures/glTexImage2D calls in UI code with Vulkan texture
uploads via new VkContext::uploadImGuiTexture() helper. This fixes segfaults
from calling OpenGL functions without a GL context (null GLEW function pointers).

- Add uploadImGuiTexture() to VkContext with staging buffer upload pattern
- Convert game_screen, inventory_screen, spellbook_screen, talent_screen
  from GLuint/GL calls to VkDescriptorSet/Vulkan uploads
- Fix loading_screen clearValueCount (was 1, needs 2 or 3 for MSAA)

include/rendering/vk_context.hpp

@@ -76,6 +76,12 @@ public:
     void setMsaaSamples(VkSampleCountFlagBits samples);
     VkSampleCountFlagBits getMaxUsableSampleCount() const;
 
+    // UI texture upload: creates a Vulkan texture from RGBA data and returns
+    // a VkDescriptorSet suitable for use as ImTextureID.
+    // The caller does NOT need to free the result — resources are tracked and
+    // cleaned up when the VkContext is destroyed.
+    VkDescriptorSet uploadImGuiTexture(const uint8_t* rgba, int width, int height);
+
 private:
     bool createInstance(SDL_Window* window);
     bool createSurface(SDL_Window* window);
@@ -144,6 +150,17 @@ private:
     VkRenderPass imguiRenderPass = VK_NULL_HANDLE;
     VkDescriptorPool imguiDescriptorPool = VK_NULL_HANDLE;
 
+    // Shared sampler for UI textures (created on first uploadImGuiTexture call)
+    VkSampler uiTextureSampler_ = VK_NULL_HANDLE;
+
+    // Tracked UI textures for cleanup
+    struct UiTexture {
+        VkImage image;
+        VkDeviceMemory memory;
+        VkImageView view;
+    };
+    std::vector<UiTexture> uiTextures_;
+
 #ifndef NDEBUG
     bool enableValidation = true;
 #else