e193f3ca88
Light has been signed off, so the theme store's default choice is now `system` (it was `dark` during the incremental migration). This matches the app.html pre-paint guard, which already resolved an unset choice via prefers-color-scheme — removing the brief boot-time mismatch where the store re-pinned dark. Users still pin light/dark via the account-menu picker. Updates the store default + its test and the design-system / finalize-plan docs. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>