Add type definitions and example .env file for environment vars

Introduce `env.d.ts` to define types for VITE environment variables, ensuring better type safety and development experience. Provide a sample `.env.example` file to guide configuration setup.

frontend/.env.example

@@ -0,0 +1,3 @@
+VITE_BASE_URL="API_URL"
+VITE_UPDATE_INTERVAL=5
+VITE_LIMIT=50

frontend/env.d.ts

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+/// <reference types="vite/client" />
+
+interface ImportMetaEnv {
+  readonly VITE_BASE_URL: string
+  readonly VITE_UPDATE_INTERVAL: number
+  readonly VITE_LIMIT: number
+}
+
+interface ImportMeta {
+  readonly env: ImportMetaEnv
+}
+
+declare module '@/*'