[B1] Cosmetic rewards: equippable titles, themes, avatar frames

Backlog. The non-inflationary reward currency: unlike XP, you can add a lot of these without unbalancing anything, and it fits an app that refuses to be a points game.

Depends on: [G2]/[G3] achievement system.

Ideas parked here

• Equippable titles shown next to the pseudo. Explicitly not one title per achievement — a title is a rare item, rarer than a badge. The 3-badge showcase already covers "show what I did"; titles cover "show what is hard to get". Membre fondateur (see [B9]) is the ultra-rare example.
• Alternative themes / accents. The "Séance" identity is built on a single ownable accent (projector amber); alternates are a natural unlock, and a clean premium candidate since they are purely cosmetic.
• Avatar frames, profile banners.

Rule

No achievement is ever behind a paywall. Cosmetics may be; rewards themselves may not.