feat(docker): add containerized nginx deployment with runtime base path

Two-stage build: node builds the static bundle, nginx serves it. The bundle is
built against a placeholder base token that the entrypoint rewrites to $BASE_URL
at start, so one image serves from any path without a rebuild.

nginx config caches hashed assets forever, never caches index.html, and applies
security headers (CSP, no framing, no referrer) suited to an app that handles
cloud credentials client-side.

.gitattributes pins LF on the container-consumed files, since core.autocrlf
would otherwise give the entrypoint a CRLF shebang.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,14 @@
+node_modules
+dist
+dist-ssr
+.git
+.gitignore
+.planning
+.claude
+.vscode
+.idea
+*.log
+*.local
+Dockerfile
+.dockerignore
+docker-compose.yml