fix(helpdesk): default period to 30 days and show trend timeframe hint

Change default period from 90 to 30 days for a more current snapshot.
Add legend footnote: '↑↓ Trend basiert auf den letzten 30 Tagen'
so the user knows what the arrows refer to.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/helpdesk/pages/helpdesk/team/index(default).phtml

@@ -34,6 +34,7 @@ $periodLabels = [
      data-label-top-categories="<?php e(t('Top categories')); ?>"
      data-label-trend-up="<?php e(t('trend_tooltip_up')); ?>"
      data-label-trend-down="<?php e(t('trend_tooltip_down')); ?>"
+     data-label-trend-hint="<?php e(t('Trend based on last {days} days')); ?>"
 >
   <section>
     <?php
@@ -100,7 +101,7 @@ $periodLabels = [
           <div data-tab-panel="performance" class="helpdesk-team-performance-panel">
             <div class="helpdesk-segment-control" id="team-period-selector">
               <?php foreach ($periods as $p): ?>
-                <input type="radio" name="team-period" id="team-period-<?php e($p); ?>" value="<?php e($p); ?>"<?php if ($p === 90): ?> checked<?php endif; ?>>
+                <input type="radio" name="team-period" id="team-period-<?php e($p); ?>" value="<?php e($p); ?>"<?php if ($p === 30): ?> checked<?php endif; ?>>
                 <label for="team-period-<?php e($p); ?>"><?php e($periodLabels[$p]); ?></label>
               <?php endforeach; ?>
             </div>