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"
     }
 }

translations/en.json

@@ -1424,5 +1424,17 @@
         "retry": "Retry",
         "syncing_local": "Syncing local data...",
         "sync_done": "Local data synced"
+    },
+    "stats_monthly": {
+        "title": "Monthly Stats",
+        "consumed": "products used",
+        "trend_up": "+{pct}% vs {prev}",
+        "trend_down": "-{pct}% vs {prev}",
+        "trend_same": "same pace as last month",
+        "added": "added",
+        "wasted": "wasted",
+        "top_used": "top used",
+        "top_cats": "Top categories",
+        "source": "Transaction history · current month"
     }
 }

translations/es.json

@@ -1367,5 +1367,17 @@
         "retry": "Reintentar",
         "syncing_local": "Sincronizando datos locales...",
         "sync_done": "Datos locales sincronizados"
+    },
+    "stats_monthly": {
+        "title": "Estadísticas Mensuales",
+        "consumed": "productos usados",
+        "trend_up": "+{pct}% vs {prev}",
+        "trend_down": "-{pct}% vs {prev}",
+        "trend_same": "mismo ritmo que el mes pasado",
+        "added": "añadidos",
+        "wasted": "desperdiciados",
+        "top_used": "más usado",
+        "top_cats": "Categorías principales",
+        "source": "Historial de transacciones · mes actual"
     }
 }

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"
     }
 }

translations/it.json

@@ -1424,5 +1424,17 @@
         "retry": "Riprova",
         "syncing_local": "Sincronizzazione dati locali...",
         "sync_done": "Dati locali aggiornati"
+    },
+    "stats_monthly": {
+        "title": "Statistiche Mensili",
+        "consumed": "prodotti usati",
+        "trend_up": "+{pct}% rispetto a {prev}",
+        "trend_down": "-{pct}% rispetto a {prev}",
+        "trend_same": "stesso ritmo del mese scorso",
+        "added": "aggiunti",
+        "wasted": "sprecati",
+        "top_used": "più usato",
+        "top_cats": "Categorie principali",
+        "source": "Storico transazioni · mese corrente"
     }
 }