Add multi-expansion support with data-driven protocol layer

Replace hardcoded WotLK protocol constants with a data-driven architecture
supporting Classic 1.12.1, TBC 2.4.3, and WotLK 3.3.5a. Each expansion
has JSON profiles for opcodes, update fields, and DBC layouts, plus C++
polymorphic packet parsers for binary format differences (movement flags,
speed fields, transport data, spline format, char enum layout).

Key components:
- ExpansionRegistry: scans Data/expansions/*/expansion.json at startup
- OpcodeTable: logical enum <-> wire values loaded from JSON
- UpdateFieldTable: field indices loaded from JSON per expansion
- DBCLayout: schema-driven DBC field lookups replacing magic numbers
- PacketParsers: WotLK/TBC/Classic parsers with correct flag positions
- Multi-manifest AssetManager: layered manifests with priority ordering
- HDPackManager: overlay texture packs with expansion compatibility
- Auth screen expansion picker replacing hardcoded version dropdown

include/auth/auth_handler.hpp

@@ -45,6 +45,10 @@ public:
     void authenticate(const std::string& username, const std::string& password);
     void authenticateWithHash(const std::string& username, const std::vector<uint8_t>& authHash);
 
+    // Set client version info (call before authenticate)
+    void setClientInfo(const ClientInfo& info) { clientInfo = info; }
+    const ClientInfo& getClientInfo() const { return clientInfo; }
+
     // Realm list
     void requestRealmList();
     const std::vector<Realm>& getRealms() const { return realms; }