Everywhere: Stop building with -fsigned-char

Ports aren't guaranteed to compile with `-fsigned-char`, resulting in
them using a different incompatible ABI when using serenity libraries.
I think we should rather follow the target architecture ABI than
assuming that `char` is signed.

Building without `-fsigned-char` only required a couple of small changes
to prevent -Wtype-limits warnings.

Documentation/QtCreatorConfiguration.md

@@ -28,7 +28,7 @@ First, make sure you have a working toolchain and can build and run SerenityOS.
 
     If you're working on the Kernel, just uncomment `#define KERNEL`.
 
--   Edit the `serenity.cxxflags` file to say `-std=c++26 -fsigned-char -fconcepts -fno-exceptions -fno-semantic-interposition -fPIC`
+-   Edit the `serenity.cxxflags` file to say `-std=c++26 -fconcepts -fno-exceptions -fno-semantic-interposition -fPIC`
 -   Edit the `serenity.includes` file to list the following lines:
     ```
     ./