Add support for Duel Commander mode

**Is your feature request related to a problem? Please describe.**
Currently Forge supports traditional Commander, but does not provide the option to play **Duel Commander**, which is a widely played competitive 1v1 format. This limits players who want to test or enjoy this variant within Forge.

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**Describe the solution you'd like**
Add support for **Duel Commander mode** with the following rules:
- 1v1 duels
- Starting life total: 20 (instead of 40)
- No commander damage rule (the 21 damage condition does not apply)
- After the second game in a match, players may swap their commander with another commander of the same color identity already included in their deck
- Use the official Duel Commander banlist: [Duel Commander Banlist](https://www.mtgdc.info/banned-restricted)

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**Describe alternatives you've considered**
- Playing regular Commander in Forge with house rules (manually tracking 20 life, ignoring commander damage, etc.), but this is cumbersome and error-prone.
- Using other platforms that support Duel Commander, but this would fragment the player base and reduce Forge’s appeal.

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**Additional context**
Duel Commander is a popular competitive format, especially in Europe, and adding it would expand Forge’s versatility. It would also align Forge with official Duel Commander rules, making it more attractive for players who want to practice or enjoy this format digitally.