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.

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;