[Feature] Free monthly cosmetic drop — login/subscriber reward

[iiamlewis]

A rotating free cosmetic each month as a retention/login reward.

Scope

• Monthly rotating free cosmetic — define eligibility (all logged-in vs subscribers only).
• Claim mechanic + "new this month" surfacing in shop.
• Inventory of drop-eligible cosmetics; never-restocked vs evergreen policy.
• Coordinate with Battle Pass ([Feature] Battle Pass system — 6-week seasons, free and paid tiers #4032) seasonal cadence to avoid overlap.

Parent: #4027