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; };