Implement Theming (Light / Dark / Brand)

Background: Native [platform] app, [SwiftUI / Compose]. I need
[light + dark mode / a custom brand theme / multiple themes the user can
pick]. Current styling: [centralized tokens / ad hoc].

Use case: Implement theming so the whole app responds to the active theme
without per-view conditionals.

Implementation details:
- Semantic color naming (e.g., "surface", "onSurface", "accent") rather
  than literal colors, so themes swap cleanly
- iOS: asset-catalog colors with light/dark variants, or a theme injected
  via the Environment; respect the system appearance and any in-app
  override. Android: theme/color scheme via MaterialTheme or a custom
  CompositionLocal; respect system dark theme + dynamic color (Material
  You) where wanted.
- A user-facing override (System / Light / Dark) persisted across launches
- Handling images/illustrations that differ per theme

Deliverable: The theming setup, the semantic token mapping per theme, the
system-vs-override resolution logic, where the choice is persisted, and a
component shown rendering correctly in each theme.