[G8] Gamified onboarding ("Première séance")

A rewarded checklist of first steps. Highest-leverage mechanic given where the product is: the problem today is not six-month retention, it is that a newcomer lands on an empty library and leaves.

Depends on: [G1] XP ledger, [G2] Achievement engine.

Scope

• A discreet progress panel — not a tutorial overlay, not a modal. A small persistent indicator the user can open, and dismiss once finished.
• Rewarded steps: add a first title, rate something, run an import, complete the profile, follow someone (the last one only when SOCIAL_ENABLED).
• Calibrated so that finishing it reaches level 2 — the curve is set up in G1 for exactly this.
• A completion achievement at the end (defined in G3).
• Side effect that is really the point: the user ends up with a non-empty library and has seen what the app does.

Open questions for implementation

• State derived from existing data, or a stored per-user checklist?
• Where the panel lives on desktop vs. the mobile PWA (check the mobile viewport, per repo convention).