feat: add Vite environment type definitions for API base URL configuration

Add TypeScript declarations for Vite environment variables including VITE_API_BASE_URL to enable type-safe access to import.meta.env in the frontend application

frontend/src/vite-env.d.ts

@@ -0,0 +1,10 @@
+/// <reference types="vite/client" />
+
+interface ImportMetaEnv {
+  readonly VITE_API_BASE_URL?: string;
+}
+
+interface ImportMeta {
+  readonly env: ImportMetaEnv;
+}
+