feat: show queued spell icon in cast bar and expose getQueuedSpellId()

When a spell is queued in the 400ms window before the current cast ends,
render its icon dimmed (0.8 alpha) to the right of the cast bar progress,
with a "Queued: <name>" tooltip. The progress bar shrinks to accommodate
the icon when one is present.

Also exposes getQueuedSpellId() as a public const accessor on GameHandler
so the UI can observe the spell queue state without friend access.

include/game/game_handler.hpp

@@ -807,6 +807,9 @@ public:
     int getCraftQueueRemaining() const { return craftQueueRemaining_; }
     uint32_t getCraftQueueSpellId() const { return craftQueueSpellId_; }
 
+    // 400ms spell-queue window: next spell to cast when current finishes
+    uint32_t getQueuedSpellId() const { return queuedSpellId_; }
+
     // Unit cast state (tracked per GUID for target frame + boss frames)
     struct UnitCastState {
         bool     casting         = false;