stats: add 1h, 4h, 3d time range windows

Adds Hour (5-min buckets), FourHour (15-min buckets), and ThreeDay
(hourly buckets) to the Window enum, plus the matching tab buttons
in stats.html. Simplifies web_ui.rs to use Window::span_secs()
instead of a duplicate match.

Closes #25

hive-ag3nt/assets/stats.html

@@ -68,7 +68,10 @@
   </div>
 
   <div class="window-tabs" id="window-tabs">
+    <button data-w="1h">last 1h</button>
+    <button data-w="4h">last 4h</button>
     <button data-w="24h" class="active">last 24h</button>
+    <button data-w="3d">last 3d</button>
     <button data-w="7d">last 7d</button>
     <button data-w="30d">last 30d</button>
   </div>