Complete light and dark themes

Add complete light and dark theme support to Screenote. The public website is currently dark while the dashboard is white; make both surfaces support both themes, including a polished dark dashboard. Default to the operating system preference via prefers-color-scheme for first-time users, provide an accessible light/dark/system theme switcher, persist explicit user choice across sessions, and react to system theme changes while System is selected. Apply semantic theme tokens consistently to navigation, forms, dialogs, tables, cards, editors, comments, answer flows, empty/loading/error states, images and media surfaces. Prevent a flash of the wrong theme on initial load, maintain contrast and focus visibility, and add automated coverage for preference resolution, persistence, switching, and representative UI states.