game/ui: add target cast bar to target frame (SMSG_SPELL_START tracking)

SMSG_SPELL_START fires for all units, not just the player. Previously only
the player's own cast was tracked; now we also track when the current
target is casting, enabling interrupt decisions.

- GameHandler: track targetCasting_/targetCastSpellId_/targetCastTimeTotal_
  /targetCastTimeRemaining_ — updated by SMSG_SPELL_START for the current
  target and ticked down in the update loop each frame
- Target cast cleared when: target changes (setTarget), target's spell
  lands (SMSG_SPELL_GO), or cast timer expires naturally
- game_screen: renderTargetFrame shows a red cast progress bar between
  the power bar and distance line when the target is casting, with
  spell name + remaining seconds
- Public accessors: isTargetCasting(), getTargetCastSpellId(),
  getTargetCastProgress(), getTargetCastTimeRemaining()

src/game/game_handler.cpp

@@ -759,6 +759,16 @@ void GameHandler::update(float deltaTime) {
             }
         }
 
+        // Tick down target cast bar
+        if (targetCasting_ && targetCastTimeRemaining_ > 0.0f) {
+            targetCastTimeRemaining_ -= deltaTime;
+            if (targetCastTimeRemaining_ <= 0.0f) {
+                targetCasting_           = false;
+                targetCastSpellId_       = 0;
+                targetCastTimeRemaining_ = 0.0f;
+            }
+        }
+
         // Update spell cooldowns (Phase 3)
         for (auto it = spellCooldowns.begin(); it != spellCooldowns.end(); ) {
             it->second -= deltaTime;
@@ -8546,6 +8556,11 @@ void GameHandler::setTarget(uint64_t guid) {
 
     targetGuid = guid;
 
+    // Clear target cast bar when target changes
+    targetCasting_           = false;
+    targetCastSpellId_       = 0;
+    targetCastTimeRemaining_ = 0.0f;
+
     // Inform server of target selection (Phase 1)
     if (state == WorldState::IN_WORLD && socket) {
         auto packet = SetSelectionPacket::build(guid);
@@ -12641,6 +12656,14 @@ void GameHandler::handleSpellStart(network::Packet& packet) {
     SpellStartData data;
     if (!packetParsers_->parseSpellStart(packet, data)) return;
 
+    // Track cast bar for the current target (for interrupt awareness)
+    if (data.casterUnit == targetGuid && data.castTime > 0) {
+        targetCasting_           = true;
+        targetCastSpellId_       = data.spellId;
+        targetCastTimeTotal_     = data.castTime / 1000.0f;
+        targetCastTimeRemaining_ = targetCastTimeTotal_;
+    }
+
     // If this is the player's own cast, start cast bar
     if (data.casterUnit == playerGuid && data.castTime > 0) {
         casting = true;
@@ -12713,6 +12736,13 @@ void GameHandler::handleSpellGo(network::Packet& packet) {
         castTimeRemaining = 0.0f;
     }
 
+    // Clear target cast bar when the target's spell lands
+    if (data.casterUnit == targetGuid) {
+        targetCasting_           = false;
+        targetCastSpellId_       = 0;
+        targetCastTimeRemaining_ = 0.0f;
+    }
+
     // Show miss/dodge/parry/etc combat text when player's spells miss targets
     if (data.casterUnit == playerGuid && !data.missTargets.empty()) {
         static const CombatTextEntry::Type missTypes[] = {

src/ui/game_screen.cpp

@@ -2093,6 +2093,22 @@ void GameScreen::renderTargetFrame(game::GameHandler& gameHandler) {
             }
         }
 
+        // Target cast bar — shown when the target is casting
+        if (gameHandler.isTargetCasting()) {
+            float castPct   = gameHandler.getTargetCastProgress();
+            float castLeft  = gameHandler.getTargetCastTimeRemaining();
+            uint32_t tspell = gameHandler.getTargetCastSpellId();
+            const std::string& castName = (tspell != 0) ? gameHandler.getSpellName(tspell) : "";
+            ImGui::PushStyleColor(ImGuiCol_PlotHistogram, ImVec4(0.9f, 0.3f, 0.2f, 1.0f));
+            char castLabel[72];
+            if (!castName.empty())
+                snprintf(castLabel, sizeof(castLabel), "%s (%.1fs)", castName.c_str(), castLeft);
+            else
+                snprintf(castLabel, sizeof(castLabel), "Casting... (%.1fs)", castLeft);
+            ImGui::ProgressBar(castPct, ImVec2(-1, 14), castLabel);
+            ImGui::PopStyleColor();
+        }
+
         // Distance
         const auto& movement = gameHandler.getMovementInfo();
         float dx = target->getX() - movement.x;