feat: add Dockerfile + nginx config for Coolify deployment

Multi-stage build: Flutter stable builds web release, nginx:alpine serves it.
nginx.conf handles SPA routing (all paths → index.html) and caches assets.

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

nginx.conf

@@ -0,0 +1,19 @@
+server {
+    listen 80;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # Flutter Web SPA — all routes go to index.html
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+
+    # Cache static assets
+    location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg|woff|woff2|ttf)$ {
+        expires 1y;
+        add_header Cache-Control "public, immutable";
+    }
+
+    gzip on;
+    gzip_types text/plain text/css application/json application/javascript text/javascript;
+}