refactor: remaining C-style casts, color constants, and header guard cleanup

Replace ~37 remaining C-style casts with static_cast across 16 files.
Extract named color constants (kColorRed/Green/Yellow/Gray) and dialog
window flags (kDialogFlags) in game_screen.cpp, replacing 72 inline
literals. Normalize keybinding_manager.hpp to #pragma once.

src/ui/auth_screen.cpp

@@ -37,7 +37,7 @@ static std::string trimAscii(std::string s) {
 static std::string hexEncode(const std::vector<uint8_t>& data) {
     std::ostringstream ss;
     for (uint8_t b : data)
-        ss << std::hex << std::setfill('0') << std::setw(2) << (int)b;
+        ss << std::hex << std::setfill('0') << std::setw(2) << static_cast<int>(b);
     return ss.str();
 }