Add UTC timestamp tooltip to Last Check time

Shows the full UTC timestamp when hovering over the relative 'time ago'
text on the equipment detail page.

Changes:
- Wrapped time_ago display in a span with title attribute
- Title shows full datetime in UTC format
- Added cursor-help class for visual hint that tooltip is available

Users can now see both the easy-to-read relative time ('5m ago') and
the precise UTC timestamp (2026-01-04 19:30:15Z) by hovering.

lib/towerops_web/live/equipment_live/show.html.heex

@@ -54,7 +54,12 @@
     <div class="bg-white dark:bg-zinc-800 rounded-lg p-3 border border-zinc-200 dark:border-zinc-700">
       <div class="text-xs text-zinc-600 dark:text-zinc-400 mb-1">Last Check</div>
       <div class="text-sm font-medium text-zinc-900 dark:text-zinc-100">
-        {time_ago(@equipment.last_checked_at)}
+        <span
+          title={if @equipment.last_checked_at, do: "#{@equipment.last_checked_at} UTC", else: nil}
+          class="cursor-help"
+        >
+          {time_ago(@equipment.last_checked_at)}
+        </span>
       </div>
     </div>
   </div>