ui: resolve usernames with getent for UID rules

[tttjhgan]

Summary

• Use getent passwd to populate local UID rule users instead of pwd.getpwall()
• Resolve typed UID usernames through pwd.getpwnam, local getent, and remote non-interactive SSH getent
• Prevent uidCombo from handling invalid indexes when users type custom text
• Fix local node detection to compare IP addresses exactly instead of substring matching

Testing

• python3 -m py_compile ui/opensnitch/dialogs/ruleseditor/dialog.py ui/opensnitch/nodes.py ui/tests/dialogs/test_ruleseditor.py ui/tests/ test_nodes.py
• git diff --check

Note: targeted pytest collection could not run in this local environment because the Python grpc module is missing.