Weekly Cocktail: server-provided combo + show its 3 component decks

[ChronoFinale]

Status: needs triage (feature / epic — server + client)

Summary

Cocktail is normally a specific weekly combination of 3 decks (from a tournament winner). In a ban/draft context a Cocktail tile is opaque — it doesn't show what it contains — and its contents currently depend on local config that isn't reliably seeded (see #485).

Proposal

• Server owns and exposes "this week's cocktail" (the 3 component decks); client fetches it.
• A Cocktail tile in the draft shows its 3 component decks (tooltip or decorated tile) so players can ban/pick it knowingly.
• This is the prerequisite for including Cocktail in the ranked pool (Ranked deck pool: curate to a fixed allowlist (fixes cross-install fairness bug) #491).

Open questions

• Whether Cocktail is included in ranked at all.
• API shape for fetching the current weekly cocktail.
• Interaction with the existing Cocktail seeding bug ([Beta v3 PvP] Cocktail Deck applies zero mixed-in decks #485).