fix(web): add the missing nav tab for the Rotation & Durations page

The durations partial (/partials/durations) has existed as a route with no
nav tab and no content panel referencing it - an orphaned page. That made
the new rotation-order UI unreachable through the interface (caught by the
owner testing on the rig; my endpoint-level tests fetched the partial by
URL and never noticed the missing entry point).

- New "Rotation" tab (fa-rotate icon, verified present in the vendored
  FA 6.0.0 css) between Display and Backup & Restore, wired exactly like
  the other tabs (#durations-content + hx-get + loadtab; loadTabContent()
  is fully generic, so no JS changes needed).
- Page heading updated from "Display Durations" to "Rotation & Durations"
  to match its content since the rotation-order card landed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEZK1P1Q1fu5pcuVrkrCFZ

web_interface/templates/v3/partials/durations.html

@@ -1,8 +1,8 @@
 {% import 'v3/partials/_macros.html' as ui %}
 <div class="bg-white rounded-lg shadow p-6">
     <div class="border-b border-gray-200 pb-4 mb-6">
-        <h2 class="text-lg font-semibold text-gray-900">Display Durations</h2>
-        <p class="mt-1 text-sm text-gray-600">Configure how long each screen is shown before switching. Values in seconds.</p>
+        <h2 class="text-lg font-semibold text-gray-900">Rotation &amp; Durations</h2>
+        <p class="mt-1 text-sm text-gray-600">Set the order plugins rotate on the display and how long each screen is shown. Durations are in seconds.</p>
     </div>
 
     {{ ui.settings_filter() }}