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/_default/imprint.html

@@ -0,0 +1,20 @@
+{{ define "main" }}
+<section class="px-[var(--pad)] pt-[clamp(36px,5vw,64px)] pb-[clamp(28px,3.5vw,48px)] border-b border-[var(--rule)]">
+  <div class="font-sans font-medium text-[10px] leading-none tracking-[.22em] uppercase text-ink-soft mb-4">Legal · Colophon</div>
+  <h1 class="font-display font-normal text-[clamp(48px,7vw,96px)] leading-[0.94] m-0 mb-4"><em>{{ .Title }}</em></h1>
+  <p class="font-serif italic text-[clamp(13px,1vw,16px)] leading-[1.65] text-ink-2 max-w-[55ch] m-0">
+    {{ .Description }}
+  </p>
+</section>
+
+<article class="px-[var(--pad)] py-[clamp(36px,5vw,64px)]">
+  <div class="max-w-[62ch] space-y-0
+    [&_h2]:font-sans [&_h2]:font-medium [&_h2]:text-[10px] [&_h2]:leading-none [&_h2]:tracking-[.22em] [&_h2]:uppercase [&_h2]:text-ink-soft [&_h2]:mt-10 [&_h2]:mb-4 [&_h2]:first:mt-0
+    [&_p]:font-serif [&_p]:text-[clamp(15px,1.05vw,17px)] [&_p]:leading-[1.72] [&_p]:text-ink-2 [&_p]:mt-3 [&_p]:mb-0
+    [&_strong]:font-sans [&_strong]:font-medium [&_strong]:text-[13px] [&_strong]:text-ink [&_strong]:tracking-[.02em]
+    [&_a]:border-b [&_a]:border-[var(--rule)] [&_a]:text-ink-2 [&_a]:transition-colors [&_a]:duration-150 hover:[&_a]:text-ink hover:[&_a]:border-ink
+    [&_hr]:border-none [&_hr]:block [&_hr]:h-px [&_hr]:bg-[var(--rule-2)] [&_hr]:mt-8 [&_hr]:mb-0">
+    {{ .Content }}
+  </div>
+</article>
+{{ end }}