Add precon starter deck selection mode

[vanja-ivancevic]

• Add Precon game mode that lets players browse and select from ~107 preconstructed decks at game start (for old border)
• Decks are grouped by edition (Mirage, Tempest, Urza's Saga, etc.) with an "All Editions" option
• Repurposes existing color/edition UI selectors as deck/set pickers when Precon mode is active
• Feature is gated on the presence of a decks/starter/precon/ folder, so vanilla and other mods are unaffected
• Standardize precon deck filenames to SET - Deck Name.dck format for edition auto-detection

[vanja-ivancevic]

Added a follow-up for Commander support:

• new Commander Precon Adventure mode
• uses decks/starter/commanderprecon/ with the same picker flow as Precon
• runs under Commander rules, so duel/editor behavior stays correct
• plane folder takes priority, with fallback to common if missing or empty

[Agetian]

This is great! I actually was about to start my own implementation and even had a rudimentary prototype, when I realized that you already completed this and even submitted it. No idea how this slipped my attention before, lol.