Clarify update source and handle missing browsers

[wizdom13]

What changed

• Adds a clear warning that users who installed NewPipe Material through IzzyOnDroid or another app store should update through the same store whenever possible.
• Explains that APKs downloaded directly from GitHub bypass the store's review and verification.
• Renames the direct-download action to Download from GitHub.
• Handles devices without a compatible browser gracefully instead of crashing when opening a release page.

Why

These changes address the feedback from the IzzyOnDroid review and implement the behavior promised in the issue response.

Validation

The branch contains only the update-dialog and browser-handling changes. CI should run automatically on the pull request.