feat(admin): redesign storage backend management UI

Implement a new card-based UI for managing storage backends in the admin panel. This update improves the visual presentation and usability of the storage configuration page.

Key changes:
- Added comprehensive CSS styles for storage cards, including status indicators, metadata layouts, and action buttons.
- Updated the storage admin template to render storage configurations as cards with type-specific details (Local, S3, SFTP, SMB, WebDAV).
- Integrated inline actions for testing, editing, disabling, and deleting storage backends.
- Enhanced sidebar link alignment with flexbox.

backend/templates/pages/admin_settings.html

@@ -4,20 +4,20 @@
 <section class="app-shell admin-shell" aria-labelledby="admin-settings-title">
   <aside class="app-sidebar">
     <nav class="sidebar-nav">
-      <a class="sidebar-link" href="/admin">Overview</a>
-      <a class="sidebar-link" href="/admin/files">Files</a>
-      <a class="sidebar-link" href="/admin/users">Users</a>
-      <a class="sidebar-link is-active" href="/admin/settings">Settings</a>
-      <a class="sidebar-link" href="/admin/storage">Storage</a>
+      <a class="sidebar-link" href="/admin">{{template "icon-dashboard" .}}<span>Overview</span></a>
+      <a class="sidebar-link" href="/admin/files">{{template "icon-folder" .}}<span>Files</span></a>
+      <a class="sidebar-link" href="/admin/users">{{template "icon-user-circle" .}}<span>Users</span></a>
+      <a class="sidebar-link is-active" href="/admin/settings">{{template "icon-settings" .}}<span>Settings</span></a>
+      <a class="sidebar-link" href="/admin/storage">{{template "icon-database" .}}<span>Storage</span></a>
     </nav>
     <hr class="sidebar-sep">
     <nav class="sidebar-nav">
-      <a class="sidebar-link" href="/app">My Files</a>
+      <a class="sidebar-link" href="/app">{{template "icon-home-simple" .}}<span>My Files</span></a>
     </nav>
     <hr class="sidebar-sep">
     <form class="sidebar-logout" action="/admin/logout" method="post">
       <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">
-      <button class="button button-outline" type="submit">Sign out</button>
+      <button class="button button-outline" type="submit">{{template "icon-log-out" .}}<span>Sign out</span></button>
     </form>
   </aside>