Preference Persistence (Theme + Locale)

The App Shell owns user preference application for theme mode and locale, so the application behaves consistently across routes and refreshes.

What is persisted?

Theme mode: light/dark
Locale: German/English

Persisting these preferences at the shell level ensures: - a single “source of truth” for UI chrome - consistent theming for pages, dialogs, and global components

Where preferences live

Preferences are owned by the active shell (public or authenticated).
Downstream components receive the current values and callbacks via props.
Internationalization is synchronized with the i18n runtime.

Persistence mechanism (conceptual)

graph LR User["User action"] --> Shell["Shell state"] Shell --> Storage["Browser storage"] Shell --> Theme["Theme build"] Shell --> I18n["i18n language"] Theme --> UI["All UI"] I18n --> UI

Architectural decisions

Preference persistence happens in the shell, not in feature code.
Theme and locale are treated as cross-cutting UI concerns, so they are not reimplemented per page.
The authenticated shell and public shell may differ in UI chrome, but preference behavior is consistent.

Not covered here

Theme token design and component styling conventions (see Theming)
Date/number formatting strategies (documented with i18n and UI conventions)