fix: hardcode apiBaseUrl to /api instead of relying on import.meta.env

VITE_API_BASE_URL may not be properly resolved during Docker build,
causing axios to construct malformed URLs.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

frontend/src/api/client.ts

@@ -3,7 +3,7 @@ import { ElNotification } from "element-plus";
 import { markBackendAvailable, markBackendUnavailable } from "@/composables/useBackendStatus";
 import { isNoBackendPreviewMode } from "@/utils/devPreview";
 
-export const apiBaseUrl = import.meta.env.VITE_API_BASE_URL ?? "/api";
+const apiBaseUrl = "/api";
 
 const apiClient = axios.create({
   baseURL: apiBaseUrl,