Fix Xtream API 403 errors by adding a fallback User-Agent (#274)

[CoderBoxer]

Overview

This pull request addresses issue #274, where Hypnotix fails to authenticate with certain Xtream API providers because the server rejects requests lacking a standard web browser User-Agent header with a 403 Forbidden error.

Changes

• Added a module-level fallback HEADERS dictionary containing a standard Chrome/Linux User-Agent string.
• Implemented a _get_headers() helper method inside the Xtream class.
• Refactored authenticate() and _get_request() to route through this helper.
• Safety Mechanism: The logic explicitly checks if self.user_agent is populated. If a user has defined a custom User-Agent via the Hypnotix preferences GUI, that preference is strictly respected. The fallback Chrome header is only applied if the custom setting is empty, preventing the application from sending a default python-requests header and triggering a 403.

Closes #274