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/de.json

@@ -1424,5 +1424,17 @@
         "retry": "Erneut versuchen",
         "syncing_local": "Lokale Daten synchronisieren...",
         "sync_done": "Lokale Daten aktualisiert"
+    },
+    "stats_monthly": {
+        "title": "Monatsstatistik",
+        "consumed": "Produkte verbraucht",
+        "trend_up": "+{pct}% vs. {prev}",
+        "trend_down": "-{pct}% vs. {prev}",
+        "trend_same": "gleiches Tempo wie letzten Monat",
+        "added": "hinzugefügt",
+        "wasted": "verschwendet",
+        "top_used": "meistbenutzt",
+        "top_cats": "Hauptkategorien",
+        "source": "Transaktionsverlauf · aktueller Monat"
     }
 }