The Light / Dark / System segmented control was previously only on the standalone Appearance tab. Users naturally look for it on the Profile page first, so it is now embedded directly in /app/account/profile alongside avatar, display name, and username. Extracted the segmented control into a single shared ThemeSegment component so the Profile form, the Appearance tab, the topbar toggle, and the sidebar/user-menu can never drift. Both surfaces auto-subscribe to OS preference changes when System is selected. Removed a personal-name placeholder from the Username input.