feat: flash action bar button red when spell cast fails

Add SpellCastFailedCallback to GameHandler, fired from SMSG_CAST_RESULT
when result != 0. GameScreen registers the callback and records each failed
spellId in actionFlashEndTimes_ (keyed by spell ID, value = expiry time).

During action bar rendering, if a slot's spell has an active flash entry,
an AddRectFilled overlay is drawn over the button with alpha proportional
to remaining time (1.0→0.0 over 0.5 s), giving the same error-red flash
visual feedback as the original WoW client.

include/game/game_handler.hpp

@@ -939,6 +939,10 @@ public:
     using SpellCastAnimCallback = std::function<void(uint64_t guid, bool start, bool isChannel)>;
     void setSpellCastAnimCallback(SpellCastAnimCallback cb) { spellCastAnimCallback_ = std::move(cb); }
 
+    // Fired when the player's own spell cast fails (spellId of the failed spell).
+    using SpellCastFailedCallback = std::function<void(uint32_t spellId)>;
+    void setSpellCastFailedCallback(SpellCastFailedCallback cb) { spellCastFailedCallback_ = std::move(cb); }
+
     // Unit animation hint: signal jump (animId=38) for other players/NPCs
     using UnitAnimHintCallback = std::function<void(uint64_t guid, uint32_t animId)>;
     void setUnitAnimHintCallback(UnitAnimHintCallback cb) { unitAnimHintCallback_ = std::move(cb); }
@@ -3309,6 +3313,7 @@ private:
     MeleeSwingCallback meleeSwingCallback_;
     uint64_t lastMeleeSwingMs_ = 0;   // system_clock ms at last player auto-attack swing
     SpellCastAnimCallback spellCastAnimCallback_;
+    SpellCastFailedCallback spellCastFailedCallback_;
     UnitAnimHintCallback unitAnimHintCallback_;
     UnitMoveFlagsCallback unitMoveFlagsCallback_;
     NpcSwingCallback npcSwingCallback_;

include/ui/game_screen.hpp

@@ -58,6 +58,10 @@ private:
     // Set to true by /stopmacro; checked in executeMacroText to halt remaining commands.
     bool macroStopped_ = false;
 
+    // Action bar error-flash: spellId → wall-clock time (seconds) when the flash ends.
+    // Populated by the SpellCastFailedCallback; queried during action bar button rendering.
+    std::unordered_map<uint32_t, float> actionFlashEndTimes_;
+
     // Tab-completion state for slash commands
     std::string chatTabPrefix_;            // prefix captured on first Tab press
     std::vector<std::string> chatTabMatches_;  // matching command list
@@ -109,6 +113,8 @@ private:
     std::vector<UIErrorEntry> uiErrors_;
     bool uiErrorCallbackSet_ = false;
     static constexpr float kUIErrorLifetime = 2.5f;
+    bool castFailedCallbackSet_ = false;
+    static constexpr float kActionFlashDuration = 0.5f;  // seconds for error-red overlay to fade
 
     // Reputation change toast: brief colored slide-in below minimap
     struct RepToastEntry { std::string factionName; int32_t delta = 0; int32_t standing = 0; float age = 0.0f; };

src/game/game_handler.cpp

@@ -2267,6 +2267,7 @@ void GameHandler::handlePacket(network::Packet& packet) {
                     std::string errMsg = reason ? reason
                                                 : ("Spell cast failed (error " + std::to_string(castResult) + ")");
                     addUIError(errMsg);
+                    if (spellCastFailedCallback_) spellCastFailedCallback_(castResultSpellId);
                     MessageChatData msg;
                     msg.type     = ChatType::SYSTEM;
                     msg.language = ChatLanguage::UNIVERSAL;

src/ui/game_screen.cpp

@@ -414,6 +414,16 @@ void GameScreen::render(game::GameHandler& gameHandler) {
         uiErrorCallbackSet_ = true;
     }
 
+    // Flash the action bar button whose spell just failed (0.5 s red overlay).
+    if (!castFailedCallbackSet_) {
+        gameHandler.setSpellCastFailedCallback([this](uint32_t spellId) {
+            if (spellId == 0) return;
+            float now = static_cast<float>(ImGui::GetTime());
+            actionFlashEndTimes_[spellId] = now + kActionFlashDuration;
+        });
+        castFailedCallbackSet_ = true;
+    }
+
     // Set up reputation change toast callback (once)
     if (!repChangeCallbackSet_) {
         gameHandler.setRepChangeCallback([this](const std::string& name, int32_t delta, int32_t standing) {
@@ -8435,6 +8445,25 @@ void GameScreen::renderActionBar(game::GameHandler& gameHandler) {
             ImGui::PopStyleColor();
         }
 
+        // Error-flash overlay: red fade on spell cast failure (~0.5 s).
+        if (slot.type == game::ActionBarSlot::SPELL && slot.id != 0) {
+            auto flashIt = actionFlashEndTimes_.find(slot.id);
+            if (flashIt != actionFlashEndTimes_.end()) {
+                float now = static_cast<float>(ImGui::GetTime());
+                float remaining = flashIt->second - now;
+                if (remaining > 0.0f) {
+                    float alpha = remaining / kActionFlashDuration;  // 1→0
+                    ImVec2 rMin = ImGui::GetItemRectMin();
+                    ImVec2 rMax = ImGui::GetItemRectMax();
+                    ImGui::GetWindowDrawList()->AddRectFilled(
+                        rMin, rMax,
+                        ImGui::ColorConvertFloat4ToU32(ImVec4(1.0f, 0.1f, 0.1f, 0.55f * alpha)));
+                } else {
+                    actionFlashEndTimes_.erase(flashIt);
+                }
+            }
+        }
+
         bool hoveredOnRelease = ImGui::IsItemHovered(ImGuiHoveredFlags_AllowWhenBlockedByActiveItem) &&
                                 ImGui::IsMouseReleased(ImGuiMouseButton_Left);