Settings page reorganized; owner icon updated; system theme now follows OS changes

The Settings page is now organized into four labeled groups — Account (Profile, Password, Sessions), Appearance (Theme), Agent (Connect, Instructions), and More (Changelog) — replacing the previous flat list of seven ungrouped cards. The ownership icon on space entries in the sidebar has been changed from a shield to a key, which reads more clearly at small size. System theme now correctly follows OS-level dark/light preference changes in real time: if your OS switches to dark mode while memsprout has "System" selected, the app theme updates immediately without requiring a page reload.