feat(engine): PostFX cycle + fixes de reinicialització i overflow

- Afegir handlePostFXCycle() amb 5 presets (vinyeta/scanlines/cromàtica/complet/off)
  i tecla X per ciclar-los (input_handler + engine.hpp)
- Augmentar MAX_SPRITES de 65536 a 200000 i afegir guard d'overflow a pushQuad()
- Netejar textures/objectes UI abans de reinicialitzar (AppLogo::initialize,
  UIManager::initialize) per evitar leaks en toggleRealFullscreen

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

source/ui/ui_manager.cpp

@@ -70,6 +70,11 @@ UIManager::~UIManager() {
 void UIManager::initialize(SDL_Renderer* renderer, ThemeManager* theme_manager,
                           int physical_width, int physical_height,
                           int logical_width, int logical_height) {
+    delete text_renderer_debug_;    text_renderer_debug_    = nullptr;
+    delete text_renderer_notifier_; text_renderer_notifier_ = nullptr;
+    delete notifier_;               notifier_               = nullptr;
+    delete help_overlay_;           help_overlay_           = nullptr;
+
     renderer_ = renderer;
     theme_manager_ = theme_manager;
     physical_window_width_ = physical_width;