[G4] Profile progression display: level, XP and the hide setting

First user-visible surface: the level itself. Kept separate from the achievements screen ([G5]) so it can ship on its own.

Depends on: [G1] XP ledger, scoring model and level curve.

Scope

• Level and progress-to-next-level on your own profile, computed client-side from stored XP through the shared pure function.
• Level shown next to the pseudo where it makes sense (reviews, comments, profile), following the same rules as the existing streak indicator.
• Privacy rules, non-negotiable: progression derives from library and social data, so its visibility follows the target's LIBRARY/ACTIVITY audience per domain, and a GHOST (Figurant) shows no level and no achievements. The precedent already exists — withStreakDays() in apps/api/src/stats/streak.util.ts refuses to attach a streak to an anonymised pseudo for exactly this reason.
• User setting "hide my progression", which turns the whole surface off for that account. Separate from the GAMIFICATION_ENABLED instance flag.
• The existing day streak is already displayed but under-exposed — give it a proper place here.

Out of scope

Achievements grid and showcase (G5), animations (G6), leaderboard (G7).