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)

translations/fr.json

@@ -1367,5 +1367,17 @@
         "retry": "Réessayer",
         "syncing_local": "Synchronisation des données locales...",
         "sync_done": "Données locales synchronisées"
+    },
+    "stats_monthly": {
+        "title": "Statistiques Mensuelles",
+        "consumed": "produits utilisés",
+        "trend_up": "+{pct}% vs {prev}",
+        "trend_down": "-{pct}% vs {prev}",
+        "trend_same": "même rythme que le mois dernier",
+        "added": "ajoutés",
+        "wasted": "gaspillés",
+        "top_used": "le plus utilisé",
+        "top_cats": "Catégories principales",
+        "source": "Historique des transactions · mois en cours"
     }
 }