[chidii]

Closes #2138

Adds 21 deterministic tests for useGamificationStore pinning down what stops a reward being paid or shown twice.

Covered:

• Level-up celebration fires once per level: not replayed after dismiss or re-check; one pending reward; multi-threshold jumps celebrate only the final level; exact-threshold and max-level boundaries; the level never lowers
• XP is credited per call (callers own idempotency); recentXPGain id behaviour that GlobalXPGain de-duplicates on, including the same-millisecond case
• Achievements: retried unlocks never add entries, unknown ids are ignored, progress updates never re-stamp an unlocked achievement
• Persistence: a reload keeps XP/level without replaying the modal or XP animation; repeated rehydration never re-credits XP; reset is persisted

The new file is added to the scoped frontend regression step in ci.yml, so it runs in CI.

Scope note: the store itself has no XP de-duplication. addXP credits every call, and dedup exists only as the guards above plus the id check in GlobalXPGain. These tests document that contract. No production code is changed here. Findings while writing them, as possible follow-ups:

1. useUserStore.updateUser calls addXP(30, "Profile completion") every time kycVerified is truthy, so a retried or refetched update pays the XP again.
2. A legacy persisted payload that still contains showLevelUpModal / pendingLevelUp / recentXPGain replays them after rehydrate (partialize only protects what is written, not what is read). Verified with a probe.
3. unlockAchievement re-stamps unlockedAt on every call (by reading the code; not asserted).
4. recentXPGain.id is Date.now(), so two rewards in one millisecond share an id and GlobalXPGain shows one animation (asserted as current behaviour).

Verification: CI's exact scoped command passes, 93 tests; prettier clean; no new tsc errors.

Note: submitted after the Oct 22 due date; please let me know if it can still count.