Projection of BrandingConfig into the flat context keys every pre-auth
template reads directly (not via branding.* dotted access): app_name,
logo_url, and the accent quad.
Embedder-provided fallback branding, attached to the router via
Extension<Arc<DefaultBranding>> when the embedder calls
AllRoutesBuilder::default_branding.
Project branding into the flat template keys every pre-auth page reads:
branding (raw, for dotted access to splash_*/forced_mode/font_*),
app_name, logo_url, and the accent quad.