Add Dockerfile and compose setup for self-hosting

Multi-stage build: Node builds the static PWA, nginx serves it with an SPA
fallback, long-lived caching for hashed assets, no-cache for the app shell
and service worker, and the correct type for the web manifest.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VsorPV3JeJRoZ1mD1pdvtL

compose.yaml

@@ -0,0 +1,8 @@
+services:
+  papure:
+    build: .
+    image: papure
+    restart: unless-stopped
+    # Put an HTTPS reverse proxy in front: the PWA install and service worker need HTTPS.
+    ports:
+      - "${PAPURE_PORT:-8080}:80"