v0.15.7: Fix backup page storage display & rename system drive label

- Add StorageBars to backupsHandler so all registered storage paths appear
- Update backups.html to use StorageBars loop (replacing single HDDConfigured block)
- Rename "SSD (/)" → "Rendszer (/)" on backup, monitoring, and dashboard pages

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

controller/internal/web/templates/dashboard.html

@@ -58,7 +58,7 @@
     <div class="system-info-items" style="margin-top: 1rem;">
         <div class="system-info-item">
             <div class="system-info-header">
-                <span class="system-info-label">SSD tárhely</span>
+                <span class="system-info-label">Rendszer (/)</span>
                 <span class="system-info-value">{{fmtGB .SystemInfo.DiskUsedGB}} / {{fmtGB .SystemInfo.DiskTotalGB}} ({{printf "%.0f" .SystemInfo.DiskPercent}}%)</span>
             </div>
             <div class="system-bar">