Add theme support (dark/light/system) #10
Description
Problem
The app currently follows the system theme but there is no way for users to override this preference.
Proposal
Add a theme setting with three options:
- System (follow device setting) — current default
- Light — always light theme
- Dark — always dark theme
Acceptance criteria
- New preference in Settings screen for theme selection
- App immediately reflects theme change without restart
- Persisted across app restarts via DataStore