ui: major UX polish and bug fixes for dashboard

- Added global loading spinner and page transitions.
- Improved sidebar with tooltips and persistent collapsed state.
- Fixed chart memory leaks by properly destroying instances on page change.
- Unified WebSocket event handling and status indicators.
- Refined stat cards, tables, and modal interactions.
- Added real backend integration for logout and session management.

static/js/pages/clients.js

@@ -77,12 +77,18 @@ class ClientsPage {
         try {
             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';
+                return;
+            }
+
             const chartData = {
                 labels: data.map(item => item.client_id),
                 datasets: [{
                     label: 'Requests',
                     data: data.map(item => item.requests),
-                    color: '#3b82f6'
+                    color: '#6366f1'
                 }]
             };