Reapply removed Blackjack enhancements after refactor

[kevin8999]

PR #100 introduced the initial implementation of Blackjack in OOP. However, it was based on an older version of the codebase. Since then, two additional PRs have extended Blackjack’s functionality (linked below for reference):

1. FEATURE / BLACKJACK - Add European Blackjack game variant ( Issue #80 ) #110
2. Added double down mechanic to blackjack game, as requested in issue #85. #96

The changes from these PRs were removed as part of PR #100 because that refactor targeted the older Blackjack implementation. These changes now need to be re-applied to the new Blackjack implementation introduced in PR #100.