feat: About dialog with version, commit, build time and settings path

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/layout_store.hpp

@@ -81,6 +81,10 @@ bool forget_source(const KnownSource& src);
 std::string math_node_text(const MathNode& node);
 bool math_node_from_text(const std::string& text, MathNode& out);
 
+/// The directory session files live in: $XDG_CONFIG_HOME/udpscope, else
+/// ~/.config/udpscope, else /tmp/udpscope-config. Not created by this call.
+std::string config_dir();
+
 /// `~/.config/udpscope/<host>_<port>.toml`, with the directory created.
 /// Characters unsafe in a filename are replaced in the host part.
 std::string layout_path(const std::string& host, std::uint16_t port);