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/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;