perf: eliminate per-frame heap allocs in M2 renderer; UI polish and report

M2 renderer: move 3 per-frame local containers to member variables:
- particleGroups_ (unordered_map): reuse bucket structure across frames
- ribbonDraws_ (vector): reuse draw call buffer
- shadowTexSetCache_ (unordered_map): reuse descriptor cache
Eliminates ~3 heap allocations per frame in particle/ribbon/shadow passes.

UI polish:
- Nameplate hover tooltip showing level, class (players), guild name
- Bag window titles show slot counts: "Backpack (12/16)"

Player report: CMSG_COMPLAIN packet builder and reportPlayer() method.
"Report Player" option in target frame right-click menu for other players.
Server response handler (SMSG_COMPLAIN_RESULT) was already implemented.

src/game/world_packets.cpp

@@ -1908,6 +1908,19 @@ network::Packet DelIgnorePacket::build(uint64_t ignoreGuid) {
     return packet;
 }
 
+network::Packet ComplainPacket::build(uint64_t targetGuid, const std::string& reason) {
+    network::Packet packet(wireOpcode(Opcode::CMSG_COMPLAIN));
+    packet.writeUInt8(1);               // complaintType: 1 = spam
+    packet.writeUInt64(targetGuid);
+    packet.writeUInt32(0);              // unk
+    packet.writeUInt32(0);              // messageType
+    packet.writeUInt32(0);              // channelId
+    packet.writeUInt32(static_cast<uint32_t>(time(nullptr))); // timestamp
+    packet.writeString(reason);
+    LOG_DEBUG("Built CMSG_COMPLAIN: target=0x", std::hex, targetGuid, std::dec, " reason=", reason);
+    return packet;
+}
+
 // ============================================================
 // Logout Commands
 // ============================================================