fix(dashboard): add COALESCE to SQL aggregations and empty-state handling for charts

Backend: wrap SUM() queries with COALESCE in handle_time_series,
handle_clients_usage, and handle_detailed_usage to prevent NULL-induced
panics when no data exists for a time window.

Frontend: add showEmptyChart() empty-state messages and error feedback
across overview, analytics, costs, and clients pages. Rewrite analytics
loadCharts() to use Promise.allSettled() so each chart renders
independently on partial API failures.

static/js/pages/clients.js

@@ -78,8 +78,7 @@ class ClientsPage {
             const data = await window.api.get('/usage/clients');
             
             if (!data || data.length === 0) {
-                const canvas = document.getElementById('client-usage-chart');
-                if (canvas) canvas.closest('.chart-container').style.display = 'none';
+                this.showEmptyChart('client-usage-chart', 'No client usage data yet');
                 return;
             }
 
@@ -96,6 +95,24 @@ class ClientsPage {
             
         } catch (error) {
             console.error('Error loading client usage chart:', error);
+            this.showEmptyChart('client-usage-chart', 'Failed to load usage data');
+        }
+    }
+
+    showEmptyChart(canvasId, message) {
+        const canvas = document.getElementById(canvasId);
+        if (!canvas) return;
+        const container = canvas.closest('.chart-container');
+        if (container) {
+            canvas.style.display = 'none';
+            let msg = container.querySelector('.empty-chart-msg');
+            if (!msg) {
+                msg = document.createElement('div');
+                msg.className = 'empty-chart-msg';
+                msg.style.cssText = 'display:flex;align-items:center;justify-content:center;height:200px;color:var(--fg4);font-size:0.9rem;';
+                container.appendChild(msg);
+            }
+            msg.textContent = message;
         }
     }