Add dark/light theming across web, desktop, and mobile surfaces

[CaptKay]

Summary

• add a reusable theme provider/toggle for the web client, share the styling with the desktop shell, and restyle nav + pages around CSS variables for dark/light palettes
• introduce Expo-native theme context + toggle, wire it through navigation, status bar, and each screen so the mobile app mirrors the desktop theming
• refresh shared loading/login UI elements to respect theme tokens and add documentation CSS for light/dark modes

Testing

• npm run lint (fails: existing lint violations in auth bootstrap files)

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Codex Task