Fix vendor buying and add quest turn-in flow

CMSG_BUY_ITEM was missing the trailing uint8 bag field, causing the
server to silently drop undersized packets. Add handlers for
SMSG_QUESTGIVER_REQUEST_ITEMS and SMSG_QUESTGIVER_OFFER_REWARD with
UI windows for quest completion and reward selection.

include/game/game_handler.hpp

@@ -363,6 +363,17 @@ public:
     bool isQuestDetailsOpen() const { return questDetailsOpen; }
     const QuestDetailsData& getQuestDetails() const { return currentQuestDetails; }
 
+    // Quest turn-in
+    bool isQuestRequestItemsOpen() const { return questRequestItemsOpen_; }
+    const QuestRequestItemsData& getQuestRequestItems() const { return currentQuestRequestItems_; }
+    void completeQuest();       // Send CMSG_QUESTGIVER_COMPLETE_QUEST
+    void closeQuestRequestItems();
+
+    bool isQuestOfferRewardOpen() const { return questOfferRewardOpen_; }
+    const QuestOfferRewardData& getQuestOfferReward() const { return currentQuestOfferReward_; }
+    void chooseQuestReward(uint32_t rewardIndex);  // Send CMSG_QUESTGIVER_CHOOSE_REWARD
+    void closeQuestOfferReward();
+
     // Quest log
     struct QuestLogEntry {
         uint32_t questId = 0;
@@ -538,6 +549,8 @@ private:
     void handleGossipMessage(network::Packet& packet);
     void handleGossipComplete(network::Packet& packet);
     void handleQuestDetails(network::Packet& packet);
+    void handleQuestRequestItems(network::Packet& packet);
+    void handleQuestOfferReward(network::Packet& packet);
     void handleListInventory(network::Packet& packet);
     LootResponseData generateLocalLoot(uint64_t guid);
     void simulateLootResponse(const LootResponseData& data);
@@ -689,6 +702,12 @@ private:
     bool questDetailsOpen = false;
     QuestDetailsData currentQuestDetails;
 
+    // Quest turn-in
+    bool questRequestItemsOpen_ = false;
+    QuestRequestItemsData currentQuestRequestItems_;
+    bool questOfferRewardOpen_ = false;
+    QuestOfferRewardData currentQuestOfferReward_;
+
     // Quest log
     std::vector<QuestLogEntry> questLog_;
 

include/game/world_packets.hpp

@@ -1238,6 +1238,62 @@ public:
     static bool parse(network::Packet& packet, QuestDetailsData& data);
 };
 
+/** Reward item entry (shared by quest detail/offer windows) */
+struct QuestRewardItem {
+    uint32_t itemId = 0;
+    uint32_t count = 0;
+    uint32_t displayInfoId = 0;
+};
+
+/** SMSG_QUESTGIVER_REQUEST_ITEMS data (turn-in progress check) */
+struct QuestRequestItemsData {
+    uint64_t npcGuid = 0;
+    uint32_t questId = 0;
+    std::string title;
+    std::string completionText;
+    uint32_t requiredMoney = 0;
+    uint32_t completableFlags = 0;  // 0x03 = completable
+    std::vector<QuestRewardItem> requiredItems;
+
+    bool isCompletable() const { return (completableFlags & 0x03) != 0; }
+};
+
+/** SMSG_QUESTGIVER_REQUEST_ITEMS parser */
+class QuestRequestItemsParser {
+public:
+    static bool parse(network::Packet& packet, QuestRequestItemsData& data);
+};
+
+/** SMSG_QUESTGIVER_OFFER_REWARD data (choose reward) */
+struct QuestOfferRewardData {
+    uint64_t npcGuid = 0;
+    uint32_t questId = 0;
+    std::string title;
+    std::string rewardText;
+    uint32_t rewardMoney = 0;
+    uint32_t rewardXp = 0;
+    std::vector<QuestRewardItem> choiceRewards;  // Pick one
+    std::vector<QuestRewardItem> fixedRewards;   // Always given
+};
+
+/** SMSG_QUESTGIVER_OFFER_REWARD parser */
+class QuestOfferRewardParser {
+public:
+    static bool parse(network::Packet& packet, QuestOfferRewardData& data);
+};
+
+/** CMSG_QUESTGIVER_COMPLETE_QUEST packet builder */
+class QuestgiverCompleteQuestPacket {
+public:
+    static network::Packet build(uint64_t npcGuid, uint32_t questId);
+};
+
+/** CMSG_QUESTGIVER_CHOOSE_REWARD packet builder */
+class QuestgiverChooseRewardPacket {
+public:
+    static network::Packet build(uint64_t npcGuid, uint32_t questId, uint32_t rewardIndex);
+};
+
 // ============================================================
 // Phase 5: Vendor
 // ============================================================

include/ui/game_screen.hpp

@@ -146,6 +146,8 @@ private:
     void renderLootWindow(game::GameHandler& gameHandler);
     void renderGossipWindow(game::GameHandler& gameHandler);
     void renderQuestDetailsWindow(game::GameHandler& gameHandler);
+    void renderQuestRequestItemsWindow(game::GameHandler& gameHandler);
+    void renderQuestOfferRewardWindow(game::GameHandler& gameHandler);
     void renderVendorWindow(game::GameHandler& gameHandler);
     void renderTeleporterPanel();
     void renderDeathScreen(game::GameHandler& gameHandler);