Add Dockerfile, nginx config, and Gitea Actions CI workflow

Multi-stage Docker build produces an Nginx container serving the
static SPA. The CI workflow runs pnpm check on every push and
builds/pushes a Docker image on semver tags.

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

nginx.conf

@@ -0,0 +1,9 @@
+server {
+    listen 80;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+}