feat(blackjack): expand player capacity to 7 and improve table visuals
- Increase maximum players from 5 to 7 (6 opponents) by updating seat positions, logic loops, and server registry.
- Add traditional felt table markings including curved text for rules ("Blackjack pays 3 to 2", "Insurance pays 2 to 1") and card/bet spot outlines.
- Refactor seat label rendering to use a consistent centered layout under portraits.
- Adjust bet circle positioning logic for the new seat layout.
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