[Stretch] Daily challenge mode with seeded shuffles

[bh2smith]

Summary

Use today's date as an RNG seed to produce a deterministic deck for any game. All players on the same day face the same deal.

Tasks

• Implement seededShuffle(deck, seed: number): PlayingCard[] in src/shared/deck.ts using a seedable PRNG (e.g. mulberry32 or xorshift — no npm package needed, ~10 lines)
• Add dailySeed(): number — converts today's date (YYYYMMDD) to a number
• Each game's deal() accepts an optional seed parameter; if provided, use seeded shuffle
• Lobby: 'Daily Challenge' button visible for each game; shares a URL with ?daily=YYYYMMDD param
• Stats panel shows daily challenge win/loss separately from free play