fix: right-clicking a quest-starting item now opens the quest offer dialog

Items with startQuestId != 0 were calling useItemBySlot()/useItemInBag()
which sends CMSG_USE_ITEM — but quest-starting items have no on-use spell,
so the server silently ignored the packet and no quest dialog appeared.

Fix:
- offerQuestFromItem(itemGuid, questId): sends CMSG_QUESTGIVER_QUERY_QUEST
  with the item's own GUID as the questgiver GUID. The server responds with
  SMSG_QUESTGIVER_QUEST_DETAILS which handleQuestDetails() already picks up
  and opens the Accept/Decline dialog with full rewards/description.
- getBagItemGuid(bagIndex, slotIndex): resolves the per-slot item GUID from
  the bag's containerContents_ map (mirrors the logic inside useItemInBag).
- inventory_screen.cpp right-click handler: checks item.startQuestId != 0
  before the equip/use branch; if set, resolves item GUID and calls
  offerQuestFromItem. Works for both backpack slots and bag slots.

include/game/game_handler.hpp

@@ -1460,6 +1460,9 @@ public:
     void acceptQuest();
     void declineQuest();
     void closeGossip();
+    // Quest-starting items: right-click triggers quest offer dialog via questgiver protocol
+    void offerQuestFromItem(uint64_t itemGuid, uint32_t questId);
+    uint64_t getBagItemGuid(int bagIndex, int slotIndex) const;
     bool isGossipWindowOpen() const { return gossipWindowOpen; }
     const GossipMessageData& getCurrentGossip() const { return currentGossip; }
     bool isQuestDetailsOpen() {