sync line added in cooperate mode

src/graphics/renderers/SyncLineRenderer.h

@@ -0,0 +1,33 @@
+#pragma once
+#include <SDL3/SDL.h>
+
+enum class SyncState {
+    Idle,
+    LeftReady,
+    RightReady,
+    Synced,
+    ClearFlash
+};
+
+class SyncLineRenderer {
+public:
+    SyncLineRenderer();
+
+    void SetRect(const SDL_FRect& rect);
+    void SetState(SyncState state);
+    void TriggerClearFlash();
+
+    void Update(float deltaTime);
+    void Render(SDL_Renderer* renderer);
+
+private:
+    SDL_FRect m_rect{};
+    SyncState m_state;
+
+    float m_flashTimer;
+    float m_time;
+
+    static constexpr float FLASH_DURATION = 0.15f;
+
+    SDL_Color GetBaseColor() const;
+};