feat(core): add achievement system with milestone unlocks and badge UI

[aiexkwan]

Summary

• Add 14 achievements unlocked by real coding milestones: first meal, level-ups (5/10/20/35), token burns (1M/10M/50M), session counts (100/500), streaks (7/14/30 days), and night owl (midnight coding)
• Achievement state stored per-pet in PetCareState.unlockedAchievements (backward-compatible Optional field)
• Pure checkAchievements(state:hour:) function + unlockNewAchievements called automatically in feedTokens/recordMeal
• Stats HUD: compact badge row with SF Symbol icons (unlocked = stage color, locked = dim)
• Care tab: achievements section listing unlocked milestones
• Celebrate animation triggers on each new unlock with achievement-specific message
• 149 lines of tests covering all unlock conditions, no-double-unlock, and backward compatibility

Test plan

• swift test — 202 tests pass, 0 failures
• swift build — clean build
• Manual: right-click pet → verify badge row appears in HUD
• Manual: feed pet → verify achievement celebrate animation fires
• Manual: check Care tab → achievements section visible