Add level-up golden burst overlay effect

When the player gains a level, a golden vignette flashes on screen edges
and a large "Level X!" text briefly appears in the center, both fading
over ~1 second. Uses the existing LevelUpCallback from GameHandler.

include/ui/game_screen.hpp

@@ -72,6 +72,8 @@ private:
     float nameplateScale_ = 1.0f; // Scale multiplier for nameplate bar dimensions
     uint32_t lastPlayerHp_ = 0;   // Previous frame HP for damage flash detection
     float damageFlashAlpha_ = 0.0f; // Screen edge flash intensity (fades to 0)
+    float levelUpFlashAlpha_ = 0.0f; // Golden level-up burst effect (fades to 0)
+    uint32_t levelUpDisplayLevel_ = 0; // Level shown in level-up text
     bool showPlayerInfo = false;
     bool showSocialFrame_ = false;  // O key toggles social/friends list
     bool showGuildRoster_ = false;
@@ -364,6 +366,7 @@ private:
     };
     std::vector<ChatBubble> chatBubbles_;
     bool chatBubbleCallbackSet_ = false;
+    bool levelUpCallbackSet_ = false;
 
     // Mail compose state
     char mailRecipientBuffer_[256] = "";

src/ui/game_screen.cpp

@@ -209,6 +209,15 @@ void GameScreen::render(game::GameHandler& gameHandler) {
         chatBubbleCallbackSet_ = true;
     }
 
+    // Set up level-up callback (once)
+    if (!levelUpCallbackSet_) {
+        gameHandler.setLevelUpCallback([this](uint32_t newLevel) {
+            levelUpFlashAlpha_ = 1.0f;
+            levelUpDisplayLevel_ = newLevel;
+        });
+        levelUpCallbackSet_ = true;
+    }
+
     // Apply UI transparency setting
     float prevAlpha = ImGui::GetStyle().Alpha;
     ImGui::GetStyle().Alpha = uiOpacity_;
@@ -700,6 +709,43 @@ void GameScreen::render(game::GameHandler& gameHandler) {
         }
     }
 
+    // Level-up golden burst overlay
+    if (levelUpFlashAlpha_ > 0.0f) {
+        levelUpFlashAlpha_ -= ImGui::GetIO().DeltaTime * 1.0f;  // fade over ~1 second
+        if (levelUpFlashAlpha_ < 0.0f) levelUpFlashAlpha_ = 0.0f;
+
+        ImDrawList* fg = ImGui::GetForegroundDrawList();
+        ImGuiIO& io = ImGui::GetIO();
+        const float W = io.DisplaySize.x;
+        const float H = io.DisplaySize.y;
+        const int alpha = static_cast<int>(levelUpFlashAlpha_ * 160.0f);
+        const ImU32 goldEdge = IM_COL32(255, 210, 50, alpha);
+        const ImU32 goldFade = IM_COL32(255, 210, 50, 0);
+        const float thickness = std::min(W, H) * 0.18f;
+
+        // Four golden gradient edges
+        fg->AddRectFilledMultiColor(ImVec2(0, 0), ImVec2(W, thickness),
+                                    goldEdge, goldEdge, goldFade, goldFade);
+        fg->AddRectFilledMultiColor(ImVec2(0, H - thickness), ImVec2(W, H),
+                                    goldFade, goldFade, goldEdge, goldEdge);
+        fg->AddRectFilledMultiColor(ImVec2(0, 0), ImVec2(thickness, H),
+                                    goldEdge, goldFade, goldFade, goldEdge);
+        fg->AddRectFilledMultiColor(ImVec2(W - thickness, 0), ImVec2(W, H),
+                                    goldFade, goldEdge, goldEdge, goldFade);
+
+        // "Level X!" text in the center during the first half of the animation
+        if (levelUpFlashAlpha_ > 0.5f && levelUpDisplayLevel_ > 0) {
+            char lvlText[32];
+            snprintf(lvlText, sizeof(lvlText), "Level %u!", levelUpDisplayLevel_);
+            ImVec2 ts = ImGui::CalcTextSize(lvlText);
+            float tx = (W - ts.x) * 0.5f;
+            float ty = H * 0.35f;
+            // Large shadow + bright gold text
+            fg->AddText(nullptr, 28.0f, ImVec2(tx + 2, ty + 2), IM_COL32(0, 0, 0, alpha), lvlText);
+            fg->AddText(nullptr, 28.0f, ImVec2(tx, ty), IM_COL32(255, 230, 80, alpha), lvlText);
+        }
+    }
+
     // Restore previous alpha
     ImGui::GetStyle().Alpha = prevAlpha;
 }