feat: add multi-stage Dockerfile for PWA

Build stage uses node:20-alpine to install deps and build.
Prod stage uses nginx:stable-alpine to serve static assets.
Includes nginx config with SPA routing, gzip, health endpoint,
and aggressive caching for Vite-hashed assets.

Closes #6

Co-Authored-By: Paperclip <noreply@paperclip.ing>

nginx.conf

@@ -0,0 +1,35 @@
+server {
+    listen 80;
+    server_name _;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # Gzip compression
+    gzip on;
+    gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript image/svg+xml;
+    gzip_min_length 256;
+
+    # Health endpoint for K8s probes
+    location /health {
+        access_log off;
+        return 200 'ok';
+        add_header Content-Type text/plain;
+    }
+
+    # Cache static assets aggressively (Vite hashes filenames)
+    location /assets/ {
+        expires 1y;
+        add_header Cache-Control "public, immutable";
+    }
+
+    # Service worker — must not be cached
+    location /sw.js {
+        expires off;
+        add_header Cache-Control "no-cache, no-store, must-revalidate";
+    }
+
+    # SPA fallback — serve index.html for all routes
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+}