gnss: add error handling to filesystem iteration in get_available_geoids

[Copilot]

get_available_geoids iterated candidate directories using std::filesystem::directory_iterator without any error handling, causing uncaught std::filesystem::filesystem_error exceptions on permission errors or broken symlinks — crashing the app at startup.

Changes

• core/src/gnss.cpp — replace all throwing filesystem calls with std::error_code overloads:
  • exists(path, ec) — avoids throw on unresolvable symlinks
  • directory_iterator(path, ec) — logs and skips the candidate path if it cannot be opened
  • dir_it.increment(ec) — logs and breaks the inner loop if a mid-iteration error occurs
  • entry.is_regular_file(ec) — silently skips entries that cannot be stat'd

std::filesystem::directory_iterator dir_it(proj_path, ec);
if (ec)
{
    std::cerr << "cannot open directory " << proj_path << ": " << ec.message() << std::endl;
    continue;
}
for (; dir_it != std::filesystem::directory_iterator(); dir_it.increment(ec))
{
    if (ec) { /* log + break */ }
    const auto& entry = *dir_it;
    if (entry.is_regular_file(ec) && !ec && entry.path().extension() == ".gtx")
        unique.insert(entry.path().filename().string());
}

Errors are reported to std::cerr; unreadable paths are skipped without interrupting the search over remaining candidates.

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