fix(render): code quality cleanup

Magic number elimination:
- Create protocol_constants.hpp, warden_constants.hpp,
  render_constants.hpp, ui_constants.hpp
- Replace ~55 magic numbers across game_handler, warden_handler,
  m2_renderer_render

Reduce nesting depth:
- Extract 5 parseEffect* methods from handleSpellLogExecute
  (max indent 52 → 16 cols)
- Extract resolveSpellSchool/playSpellCastSound/playSpellImpactSound
  from 3× duplicate audio blocks in handleSpellGo
- Flatten SMSG_INVENTORY_CHANGE_FAILURE with early-return guards
- Extract drawScreenEdgeVignette() for 3 duplicate vignette blocks

DRY extract patterns:
- Replace 12 compound expansion checks with isPreWotlk() across
  movement_handler (9), chat_handler (1), social_handler (1)

const to constexpr:
- Promote 23+ static const arrays/scalars to static constexpr across
  12 source files

Error handling:
- Convert PIN auth from exceptions to std::optional<PinProof>
- Add [[nodiscard]] to 15+ initialize/parse methods
- Wrap ~20 unchecked initialize() calls with LOG_WARNING/LOG_ERROR

Signed-off-by: Pavel Okhlopkov <pavel.okhlopkov@flant.com>

include/auth/auth_handler.hpp

@@ -40,7 +40,7 @@ public:
     ~AuthHandler();
 
     // Connection
-    bool connect(const std::string& host, uint16_t port = 3724);
+    [[nodiscard]] bool connect(const std::string& host, uint16_t port = 3724);
     void disconnect();
     bool isConnected() const;
 

include/auth/auth_packets.hpp

@@ -53,7 +53,7 @@ struct LogonChallengeResponse {
 // LOGON_CHALLENGE response parser
 class LogonChallengeResponseParser {
 public:
-    static bool parse(network::Packet& packet, LogonChallengeResponse& response);
+    [[nodiscard]] static bool parse(network::Packet& packet, LogonChallengeResponse& response);
 };
 
 // LOGON_PROOF packet builder
@@ -92,7 +92,7 @@ struct LogonProofResponse {
 // LOGON_PROOF response parser
 class LogonProofResponseParser {
 public:
-    static bool parse(network::Packet& packet, LogonProofResponse& response);
+    [[nodiscard]] static bool parse(network::Packet& packet, LogonProofResponse& response);
 };
 
 // Realm data structure
@@ -131,7 +131,7 @@ struct RealmListResponse {
 class RealmListResponseParser {
 public:
     // protocolVersion: 3 = vanilla (uint8 realmCount, uint32 icon), 8 = WotLK (uint16 realmCount, uint8 icon)
-    static bool parse(network::Packet& packet, RealmListResponse& response, uint8_t protocolVersion = 8);
+    [[nodiscard]] static bool parse(network::Packet& packet, RealmListResponse& response, uint8_t protocolVersion = 8);
 };
 
 } // namespace auth

include/auth/pin_auth.hpp

@@ -2,6 +2,7 @@
 
 #include <array>
 #include <cstdint>
+#include <optional>
 #include <string>
 
 namespace wowee {
@@ -19,9 +20,11 @@ struct PinProof {
 // - Compute: pin_hash = SHA1(client_salt || SHA1(server_salt || randomized_pin_ascii))
 //
 // PIN must be 4-10 ASCII digits.
-PinProof computePinProof(const std::string& pinDigits,
-                         uint32_t pinGridSeed,
-                         const std::array<uint8_t, 16>& serverSalt);
+// Returns std::nullopt on invalid input (bad length, non-digit chars, or grid corruption).
+[[nodiscard]] std::optional<PinProof> computePinProof(
+    const std::string& pinDigits,
+    uint32_t pinGridSeed,
+    const std::array<uint8_t, 16>& serverSalt);
 
 } // namespace auth
 } // namespace wowee