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/orval.config.ts

@@ -1,7 +1,6 @@
 import { defineConfig } from "orval";
-import "dotenv/config";
 
-const backendUrl = process.env.VITE_BACKEND_URL || "http://localhost:8000";
+const backendUrl = import.meta.env.VITE_BACKEND_URL || "http://localhost:8000";
 
 // může se hodit pokud nechceme při buildu generovat klienta (nechat false pro produkci nebo vynechat)
 const SKIP_ORVAL = process.env.SKIP_ORVAL === "true";