refactor: update URLs and enhance checks for fab.com integration

[apo-mak]

This pull request updates the unrealengine.js script to support claiming free assets from the new Fab.com platform instead of the old Unreal Engine Marketplace. The logic for detecting, claiming, and tracking free items has been rewritten to adapt to Fab.com's UI and flow, including improved login detection, handling of license options, and more robust error handling.

Migration to Fab.com and Claim Flow Overhaul:

• Updated URLs and login flow to use Fab.com instead of Unreal Engine Marketplace, including proper URL encoding for redirects.
• Rewrote the detection and claiming logic to:
  • Identify free items via new selectors and page structure,
  • Handle both Professional and Personal license options, prioritizing free Professional licenses when available,
  • Check if items are already in the user's library and skip claiming if so,
  • Click the appropriate "Buy now" button and confirm success by checking for "Saved in My Library".

Improved Login and User Detection:

• Enhanced login status checks to work with both Epic Games and Fab.com by checking for relevant navigation elements or library links.
• Improved user identification with a fallback when display name is unavailable. [1] [2]

Error Handling and Notifications:

• Added better error handling for claim failures, including taking screenshots with more descriptive filenames and updating statuses in the database and notifications accordingly.

Miscellaneous Improvements:

• Added checks to avoid duplicate screenshots and improved messaging throughout the script for clarity.