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

**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 (#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 (#485).