Add text input support: safe uint16_t* to wstring conversion

Replace unsafe (wchar_t*)pchText casts with uint16_to_wstring() in all
keyboard callbacks. The direct cast is incorrect on platforms where
wchar_t is 4 bytes (Linux/macOS). New helpers in StringHelpers route
through u16string for proper UTF-16 to wchar_t conversion.

- Add uint16_len, uint16_to_u16string, uint16_to_wstring to StringHelpers
- Fix casts in AnvilMenu, CreateWorldMenu, DebugCreateSchematic,
  DebugSetCamera, LaunchMoreOptionsMenu, SignEntryMenu
- Truncate sign text to 15 chars in SignEntryMenu
- Move m_bIgnoreInput reset after if-block in LaunchMoreOptionsMenu

Minecraft.Client/Platform/Common/UI/UIScene_DebugCreateSchematic.cpp

@@ -146,7 +146,7 @@ int UIScene_DebugCreateSchematic::KeyboardCompleteCallback(void* lpParam,
     InputManager.GetText(pchText);
 
     if (pchText[0] != 0) {
-        std::wstring value = (wchar_t*)pchText;
+        std::wstring value = uint16_to_wstring(pchText);
         int iVal = 0;
         if (!value.empty()) iVal = _fromString<int>(value);
         switch (pClass->m_keyboardCallbackControl) {