Switches the Jinja base layout to _modal_partial.html (yield-only) when
the HX-Modal:1 request header is set, otherwise base.html. Foundation
for URL-addressable modals (spec 2026-05-17-url-addressable-modals).
Guards with has_request_context() so the processor is safe when
render_template_string is called from app_context() without a request
(e.g. test_timeago_filter_registered_on_app).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
