Add imprint page and header nav link

Editorial prose layout with legal notice, copyright, AI training
exclusion, and analytics disclosure. Valknar / valknar@pivoine.art
as responsible person. Imprint link added to right nav (hidden
below 820 px breakpoint).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

layouts/partials/header.html

@@ -27,6 +27,8 @@
                 max-[620px]:gap-[10px] max-[620px]:text-[10px] max-[460px]:hidden">
       <a href="/issues/" class="nav-link">Issues</a>
       <span class="opacity-50 max-[820px]:hidden">·</span>
+      <a href="/imprint/" class="nav-link whitespace-nowrap max-[820px]:hidden">Imprint</a>
+      <span class="opacity-50 max-[1100px]:hidden">·</span>
       <span class="whitespace-nowrap max-[1100px]:hidden">Paris</span>
     </div>
   </div>