fix: config path advisory ignores XDG file on interactive root (#81)

Port-53 and TLS-data-dir advisories told users to create
~/.config/numa/numa.toml, but config_dir() routed root to
/var/lib/numa/ and load_config never consulted the XDG path, so
the file the user created was silently ignored.

New suggested_config_path() helper prefers $HOME/.config/numa/
when HOME is set (and isn't "/" or empty), with config_dir() as
lazy fallback. Used by both advisories and by load_config as an
additional candidate, so the advised path is the path numa
actually reads. Runtime state (services.json, TLS CA) stays in
FHS — config_dir()/data_dir() are intentionally unchanged to
keep continuity with the installed daemon.

End-to-end replication + regression check in
tests/docker/issue-81.sh: four scenarios (replication and
existing-install, each against main and fix), all matching
expectations.

tests/docker/hold53.py

@@ -0,0 +1,5 @@
+import socket, signal
+s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 0)
+s.bind(("", 53))
+signal.pause()