feat(web): add Analytics page with Chart.js charts and activity feed

Add Analytics page with stat cards (open tasks, active time, top category),
Chart.js timeline bar chart bucketed by hour, category donut chart with
legend, and paginated activity feed with htmx "Load more" support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

TaskTracker.Api/Pages/Partials/_ActivityFeedItems.cshtml

@@ -0,0 +1,15 @@
+@using TaskTracker.Api.Pages
+@model AnalyticsModel
+
+@foreach (var item in Model.ActivityItems)
+{
+    <div class="activity-item">
+        <span class="activity-dot" style="background: var(--color-accent)"></span>
+        <div class="activity-line"></div>
+        <div class="activity-info">
+            <span class="activity-app">@item.AppName</span>
+            <span class="activity-title">@(string.IsNullOrEmpty(item.Url) ? item.WindowTitle : item.Url)</span>
+            <span class="activity-time">@AnalyticsModel.FormatRelativeTime(item.Timestamp)</span>
+        </div>
+    </div>
+}