New centred hero component for the Briefwechsel page: headline
"Wessen Briefe möchten Sie lesen?", cross-link to document search,
h-14 PersonTypeahead, and recent persons chips. Adds `large` prop
to PersonTypeahead and `conv_hero_crosslink` message key.
Refs: #179
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
