Updated to C++23 with some minor refactoring and documentation

[Ataba29]

This PR begins the C++23 refactor of ByteForge, starting with the networking and startup layer. main.cpp's server-accept thread now uses std::jthread instead of std::thread, removing the need for a manual .join() call since it joins automatically on destruction. Server::start() now returns std::expected<void, std::string> instead of throwing std::runtime_error on bind/listen failure, giving main.cpp explicit control over the failure message and exit path rather than relying on an unhandled exception. Server's connections and busySockets members were converted from std::unordered_map/std::unordered_set to std::flat_map/std::flat_set, trading hash-bucket lookups for cache-friendlier contiguous storage, which suits their typically small size. A [[assume(bytesReceived > 0)]] hint was also added in messageHandler immediately after the two early-return branches that already rule out zero and negative values, letting the compiler skip re-deriving that invariant.

Related to #19