feat: landing page — minimal dark one-pager with Dockerfile

Single static HTML page for RückenfrAI ("KI, die dir den Rücken
freihält"). Dark theme, inline CSS, Inter font. Sections: hero with
terminal mockup, features (email/voice/calendar/tasks), how it works,
waitlist CTA (mailto). nginx Dockerfile + docker-compose for Dokploy.

docker-compose.yml

@@ -0,0 +1,6 @@
+services:
+  web:
+    build: .
+    ports:
+      - "3000:80"
+    restart: unless-stopped