fix(web): show empty-state message when analytics charts have no data

Charts previously rendered as blank boxes when no context events
existed in the selected range, which looked broken.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

TaskTracker.Api/Pages/Analytics.cshtml

@@ -126,8 +126,19 @@
 
 @section Scripts {
 <script>
+    function showChartEmptyState(canvasId, message) {
+        var canvas = document.getElementById(canvasId);
+        var container = canvas.closest('.chart-container');
+        container.innerHTML = '<div class="search-empty">' + message + '</div>';
+        container.style.display = 'flex';
+        container.style.alignItems = 'center';
+        container.style.justifyContent = 'center';
+    }
+
     // Timeline bar chart
-    if (timelineData.length > 0) {
+    if (timelineData.length === 0) {
+        showChartEmptyState('timeline-chart', 'No activity recorded in this time range.');
+    } else {
         new Chart(document.getElementById('timeline-chart'), {
             type: 'bar',
             data: {
@@ -169,7 +180,9 @@
     }
 
     // Category donut chart
-    if (categoryData.length > 0) {
+    if (categoryData.length === 0) {
+        showChartEmptyState('category-chart', 'No categorized activity in this time range.');
+    } else {
         new Chart(document.getElementById('category-chart'), {
             type: 'doughnut',
             data: {