Redesign anti-waste section: report card with grade, comparison vs national avg, savings badges and trend chart

- Replace simple bar chart with full Anti-Waste Report Card
- Grade system (A+ to D) based on user's waste rate
- Dual comparison bars: user waste rate vs national average (IT/DE/US)
- Estimated monthly savings in money, meals saved, CO2 avoided
- 3-month trend mini chart with colour-coded bars
- Backend: getStats() now returns 3×30d buckets (used_30d, used_prev_30d, used_prev_60d, etc.)
- Real-world benchmarks: IT 22%/5.4kg/mo (REDUCE), DE 20%/6.5kg/mo (Eurostat), US 30%/9.2kg/mo (USDA)
- All labels fully i18n: 18 new antiwaste.* keys in it/en/de translation files
- Section is fully JS-rendered; HTML now just an empty container

index.html

@@ -96,12 +96,8 @@
             <div id="expiring-list"></div>
         </div>
 
-        <!-- Waste vs consumption mini chart -->
-        <div class="waste-chart-section" id="waste-chart-section" style="display:none">
-            <h3 data-i18n="dashboard.stats_period">📊 Ultimi 30 giorni</h3>
-            <div class="waste-chart-bar" id="waste-chart-bar"></div>
-            <div class="waste-chart-legend" id="waste-chart-legend"></div>
-        </div>
+        <!-- Anti-Waste Report Card (content fully rendered by JS) -->
+        <div id="waste-chart-section" style="display:none"></div>
 
         <!-- Opened (partially used) products -->
         <div class="alert-section alert-opened" id="alert-opened" style="display:none">