Add gameplay systems: combat, spells, groups, loot, vendors, and UI

Implement ~70 new protocol opcodes across 5 phases while maintaining
full 3.3.5a private server compatibility:

- Phase 1: Server-aware targeting (CMSG_SET_SELECTION), player/creature
  name queries, CMSG_SET_ACTIVE_MOVER after login
- Phase 2: Auto-attack, melee/spell damage parsing, health/mana/power
  tracking from UPDATE_OBJECT fields, floating combat text
- Phase 3: Spell casting, action bar (12 slots, keys 1-=), cast bar,
  cooldown tracking, aura/buff system with cancellation
- Phase 4: Group invite/accept/decline/leave, party frames UI,
  /invite chat command
- Phase 5: Loot window, NPC gossip dialog, vendor buy/sell interface

Also: disable debug HUD/panels by default, gate 3D rendering to
IN_GAME state only, fix window resize not updating UI positions.

include/game/group_defines.hpp

@@ -0,0 +1,72 @@
+#pragma once
+
+#include <cstdint>
+#include <string>
+#include <vector>
+
+namespace wowee {
+namespace game {
+
+/**
+ * Party/Group member data
+ */
+struct GroupMember {
+    std::string name;
+    uint64_t guid = 0;
+    uint8_t isOnline = 0;      // 0 = offline, 1 = online
+    uint8_t subGroup = 0;      // Raid subgroup (0 for party)
+    uint8_t flags = 0;         // Assistant, main tank, etc.
+    uint8_t roles = 0;         // LFG roles (3.3.5a)
+};
+
+/**
+ * Full group/party data from SMSG_GROUP_LIST
+ */
+struct GroupListData {
+    uint8_t groupType = 0;       // 0 = party, 1 = raid
+    uint8_t subGroup = 0;
+    uint8_t flags = 0;
+    uint8_t roles = 0;
+    uint8_t lootMethod = 0;      // 0=free for all, 1=round robin, 2=master loot
+    uint64_t looterGuid = 0;
+    uint8_t lootThreshold = 0;
+    uint8_t difficultyId = 0;
+    uint8_t raidDifficultyId = 0;
+    uint32_t memberCount = 0;
+    std::vector<GroupMember> members;
+    uint64_t leaderGuid = 0;
+
+    bool isValid() const { return true; }
+    bool isEmpty() const { return memberCount == 0; }
+};
+
+/**
+ * Party command types
+ */
+enum class PartyCommand : uint32_t {
+    INVITE = 0,
+    UNINVITE = 1,
+    LEAVE = 2,
+    SWAP = 3
+};
+
+/**
+ * Party command result codes
+ */
+enum class PartyResult : uint32_t {
+    OK = 0,
+    BAD_PLAYER_NAME = 1,
+    TARGET_NOT_IN_GROUP = 2,
+    TARGET_NOT_IN_INSTANCE = 3,
+    GROUP_FULL = 4,
+    ALREADY_IN_GROUP = 5,
+    NOT_IN_GROUP = 6,
+    NOT_LEADER = 7,
+    PLAYER_WRONG_FACTION = 8,
+    IGNORING_YOU = 9,
+    LFG_PENDING = 12,
+    INVITE_RESTRICTED = 13
+};
+
+} // namespace game
+} // namespace wowee