fix: use bracket notation for process.env access (TS5.9 strict)

- src/main.server.ts: process.env['SSR_API_BASE_URL'] instead of
  process.env.SSR_API_BASE_URL to satisfy TS4111 index signature rule.
- server.spec.ts: update corresponding test assertions to match new syntax.

src/main.server.ts

@@ -9,7 +9,7 @@ import { serverRoutes } from './app/app.server.routes';
 
 // During SSR, the API base URL must be absolute (HttpClient runs in Node.js).
 // nginx proxies /api/ to the upstream, so point at nginx (port 80) locally.
-const ssrApiBaseUrl = process.env.SSR_API_BASE_URL || 'http://localhost:80';
+const ssrApiBaseUrl = process.env['SSR_API_BASE_URL'] || 'http://localhost:80';
 const ssrAppConfig: AppConfig = { apiBaseUrl: ssrApiBaseUrl };
 
 const serverAppConfig = {