Add imprint page with privacy/attribution info and footer link

Adds /imprint/ with operator contact, self-hosted Umami analytics
disclosure, content attribution, and disclaimer sections. Links
Imprint in the footer bottom bar next to the pivoine.art credit.

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

content/imprint.md

@@ -0,0 +1,5 @@
+---
+title: "Imprint"
+description: "Legal notice and privacy information for Bar Pivoine."
+layout: "imprint"
+---