game/ui: generalize cast tracking to per-GUID map; add boss cast bars

Previously the target cast bar tracked a single target using 4 private
fields. This replaces that with unitCastStates_ (unordered_map<uint64_t,
UnitCastState>), tracking cast state for every non-player unit whose
SMSG_SPELL_START we receive.

Changes:
- GameHandler::UnitCastState struct: casting, spellId, timeRemaining,
  timeTotal
- getUnitCastState(guid) → returns cast state for any tracked unit
- isTargetCasting(), getTargetCastSpellId(), getTargetCastProgress(),
  getTargetCastTimeRemaining() now delegate to getUnitCastState(targetGuid)
- handleSpellStart: tracks all non-player casters (not just the target)
- handleSpellGo: erases caster from map when spell lands
- update loop: ticks down all unit cast states, erasing expired entries
- unitCastStates_ cleared on world reset
- renderBossFrames: shows red cast progress bar per boss slot with
  spell name + remaining seconds — critical for instance interrupt play

src/ui/game_screen.cpp

@@ -5010,6 +5010,24 @@ void GameScreen::renderBossFrames(game::GameHandler& gameHandler) {
                 ImGui::PopStyleColor();
             }
 
+            // Boss cast bar — shown when the boss is casting (critical for interrupt)
+            if (auto* cs = gameHandler.getUnitCastState(bs.guid)) {
+                float castPct  = (cs->timeTotal > 0.0f)
+                    ? (cs->timeTotal - cs->timeRemaining) / cs->timeTotal : 0.0f;
+                uint32_t bspell = cs->spellId;
+                const std::string& bcastName = (bspell != 0)
+                    ? gameHandler.getSpellName(bspell) : "";
+                ImGui::PushStyleColor(ImGuiCol_PlotHistogram, ImVec4(0.9f, 0.3f, 0.2f, 1.0f));
+                char bcastLabel[72];
+                if (!bcastName.empty())
+                    snprintf(bcastLabel, sizeof(bcastLabel), "%s (%.1fs)",
+                             bcastName.c_str(), cs->timeRemaining);
+                else
+                    snprintf(bcastLabel, sizeof(bcastLabel), "Casting... (%.1fs)", cs->timeRemaining);
+                ImGui::ProgressBar(castPct, ImVec2(-1, 12), bcastLabel);
+                ImGui::PopStyleColor();
+            }
+
             ImGui::PopID();
             ImGui::Spacing();
         }