Refactor API hooks to remove infinite query support and update API client base URL

- Removed infinite query options and related functions from Trading212, User Registration, and User API files.
- Updated API client base URLs in privateClient.ts and publicClient.ts to use environment variable for backend URL.
- Refactored Downloader component to directly call API functions for video info retrieval instead of using a hook.

frontend/src/api/privateClient.ts

@@ -1,8 +1,11 @@
 import axios, { type AxiosRequestConfig } from "axios";
 
+const backendUrl = import.meta.env.VITE_BACKEND_URL || "http://localhost:8000";
+
+
 // použij tohle pro API vyžadující autentizaci
 export const privateApi = axios.create({
-  baseURL: "/api/",
+  baseURL: backendUrl,
   withCredentials: true, // potřebuje HttpOnly cookies
 });