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>

Dockerfile

@@ -88,8 +88,10 @@ COPY --from=cm6-build /build/dist/ /usr/share/nginx/html/vendor/codemirror/
 # Builtin extensions (seeded into DB on startup by backend)
 COPY extensions/builtin/ /app/extensions/builtin/
 
-# nginx config
-COPY nginx.conf /etc/nginx/conf.d/default.conf
+# nginx config (template — envsubst injects BASE_PATH at runtime)
+COPY nginx.conf.template /etc/nginx/templates/default.conf.template
+# Only substitute BASE_PATH — preserve nginx variables ($host, $uri, etc.)
+ENV NGINX_ENVSUBST_FILTER="BASE_PATH"
 
 # Entrypoint: start Go backend, then nginx
 COPY docker-entrypoint.sh /docker-entrypoint.d/90-start-backend.sh