fix: return UINT32_MAX from findMemType on failure, add [[nodiscard]]

The findMemType/findMemoryType helper in auth_screen, loading_screen,
and vk_context returned 0 on failure — a valid memory type index.
Changed to return UINT32_MAX and log an error, so vkAllocateMemory
receives an invalid index and fails cleanly rather than silently
using the wrong memory type.

Add [[nodiscard]] to VkBuffer::uploadToGPU/createMapped and
VkContext::initialize/recreateSwapchain so callers that ignore
failure are flagged at compile time. Suppress with (void) cast at
3 call sites where failure is non-actionable (resize best-effort).

include/rendering/vk_context.hpp

@@ -32,11 +32,11 @@ public:
     VkContext(const VkContext&) = delete;
     VkContext& operator=(const VkContext&) = delete;
 
-    bool initialize(SDL_Window* window);
+    [[nodiscard]] bool initialize(SDL_Window* window);
     void shutdown();
 
     // Swapchain management
-    bool recreateSwapchain(int width, int height);
+    [[nodiscard]] bool recreateSwapchain(int width, int height);
 
     // Frame operations
     VkCommandBuffer beginFrame(uint32_t& imageIndex);