feat: monthly stats panel in rotating insight banner (closes #100)
- Add PHP action 'monthly_stats' + getMonthlyStats() function:
items consumed/added/wasted this month, trend vs prev month,
top 5 categories and top 3 products by transaction count
- Expand insight rotation from 2 → 3 phases (waste → nutrition → monthly)
- Add _renderMonthlyStatsSection() following nutrition section styling:
large indigo number, trend arrow with %, horizontal category bars
animated on show, badges for added/wasted/top-product
- Add ms-* CSS classes (bar chart layout matching nutr-* design)
- Add stats_monthly translations in it/en/de/fr/es (10 keys each)
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