fix BASE_PATH routing: nginx template + runtime HTML injection

- nginx.conf → nginx.conf.template with ${BASE_PATH} in all locations
- NGINX_ENVSUBST_FILTER=BASE_PATH so nginx vars ($host, $uri) are preserved
- Entrypoint injects BASE_PATH into frontend HTML/JS (%%BASE_PATH%% → actual)
- Fixes: /test routes falling through to static SPA, %%BASE_PATH%% visible in URL

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nginx.conf.template

@@ -0,0 +1,101 @@
+server {
+    listen 80;
+    server_name _;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # Backend upstream (unified container: localhost:8080)
+    set $backend http://127.0.0.1:8080;
+
+    # Gzip
+    gzip on;
+    gzip_vary on;
+    gzip_min_length 1024;
+    gzip_proxied expired no-cache no-store private auth;
+    gzip_types text/plain text/css text/xml text/javascript application/x-javascript application/xml application/javascript;
+
+    # Static assets — images/fonts get long cache; JS/CSS use revalidation
+    location ~* \.(jpg|jpeg|png|gif|ico|woff|woff2)$ {
+        expires 1y;
+        add_header Cache-Control "public, immutable";
+    }
+    location ~* \.(css|js)$ {
+        add_header Cache-Control "no-cache, must-revalidate";
+    }
+
+    # ── API + WebSocket → backend ────────────
+    location ^~ ${BASE_PATH}/api/ {
+        proxy_pass $backend;
+        proxy_http_version 1.1;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+    }
+
+    location = ${BASE_PATH}/health {
+        proxy_pass $backend;
+    }
+
+    location = ${BASE_PATH}/metrics {
+        proxy_pass $backend;
+    }
+
+    location = ${BASE_PATH}/ws {
+        proxy_pass $backend;
+        proxy_http_version 1.1;
+        proxy_set_header Upgrade $http_upgrade;
+        proxy_set_header Connection "upgrade";
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_read_timeout 86400s;
+        proxy_send_timeout 86400s;
+    }
+
+    # ── Page routes → backend (Go renders HTML) ─
+    location = ${BASE_PATH}/ {
+        proxy_pass $backend;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+    }
+
+    location = ${BASE_PATH}/login {
+        proxy_pass $backend;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+    }
+
+    location ${BASE_PATH}/admin  { proxy_pass $backend; proxy_set_header Host $host; proxy_set_header X-Real-IP $remote_addr; }
+    location ${BASE_PATH}/team-admin { proxy_pass $backend; proxy_set_header Host $host; proxy_set_header X-Real-IP $remote_addr; }
+    location ${BASE_PATH}/settings { proxy_pass $backend; proxy_set_header Host $host; proxy_set_header X-Real-IP $remote_addr; }
+    location ${BASE_PATH}/w/     { proxy_pass $backend; proxy_set_header Host $host; proxy_set_header X-Real-IP $remote_addr; }
+
+    # Extension surface page routes → backend
+    location ${BASE_PATH}/s/ {
+        proxy_pass $backend;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+    }
+
+    # Extension surface static assets
+    location ^~ ${BASE_PATH}/surfaces/ {
+        alias /data/storage/packages/;
+        expires off;
+        add_header Cache-Control "no-cache";
+    }
+
+    # Fallback: serve frontend shell (SPA)
+    location ${BASE_PATH}/ {
+        try_files $uri $uri/ ${BASE_PATH}/index.html;
+    }
+
+    # Security headers
+    add_header X-Frame-Options "SAMEORIGIN" always;
+    add_header X-Content-Type-Options "nosniff" always;
+    add_header X-XSS-Protection "1; mode=block" always;
+}