Fix UI window centering, python requirements and resource links broke…

[OskarYescas]

Based on customer feedback, this PR addresses Ui changes.

1. Documentation Updates (README.md)
   Added an explicit note to the "Python Version" requirements section advising users to avoid Python Beta or Release Candidate (RC) versions, as they were observed causing installation errors for dependencies.

2. Window Centering (ui/utils.py, migration_planner.py, ui/exchange_online_ui.py, ui/chats_ui.py)
   Created a lightweight center_window helper function in ui/utils.py.
   The main selector window and all three workload estimator windows (Exchange, Chat, Files) now consistently launch dead-center on the user's screen rather. Previously, this caused some misplacements if using different monitors.

3. Resource Links Fix (ui/exchange_online_ui.py, ui/chats_ui.py)
   Fixed an issue where clicking the "Learn more" resource links did nothing.
   Imported the missing webbrowser standard library and successfully bound the webbrowser.open() action.
   Added cursor="hand2" to the link labels to provide immediate visual feedback that the text is clickable.
   Currerntly, the redirect links are the same (should be updated).

Testing
Successfully tested all three UI views locally on macOS.
Verified that links open the system's default browser correctly.
Verified that all screens center flawlessly upon launch.

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