Fix GCC 14.3.0 compilation error in shader_utils.cpp

[Copilot]

GCC 14.3.0's improved static analysis flags potential null pointer dereferences in std::istreambuf_iterator usage with -Werror, causing build failures in the Nix flake CI workflow.

Changes

• Replace std::istreambuf_iterator with std::stringstream in readShaderSource()
  • Avoids false positive null-dereference warnings from GCC 14.3.0
  • Maintains identical functionality with cleaner, more idiomatic code

// Before: Triggers GCC 14.3.0 warning
return std::string((std::istreambuf_iterator<char>(file)),
                   std::istreambuf_iterator<char>());

// After: Standard idiom, no warnings
std::stringstream buffer;
buffer << file.rdbuf();
return buffer.str();

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[jamylak]

@copilot I didn't want you to switch to stringstream, i wanted you to leave it as it was before, but make sure the flake is using CPP 20