refactor: modernize SensirionI2cSht4x code style and fix implicit conversions

[Chris0xdeadbeef]

I found several code quality issues in SensirionI2cSht4x.h and .cpp files:

SensirionI2cSht4x.h:

• Remove redundant static from file-scope constexpr constants
• Remove redundant #ifndef include guard, #pragma once is sufficient
• Replace non-standard GCC typeof with standard C++ decltype

SensirionI2cSht4x.cpp:

• Replace typedef enum (C style) with typed enum : uint8_t (C++)
• Replace magic number command IDs with SHT4xCmdId enum values
• Replace implicit double literals with float suffix
• Simplify signal conversion functions to single return statement

No functional change.