refactor(ui): remaster settings and navigation layout

- Update navigation labels from "My Account" to "Dashboard" and "Login" to "Sign in", updating tests accordingly.
- Redesign settings forms into structured sections with improved spacing and layout.
- Add CSS styles for tabs, small buttons, and responsive settings sections to enhance the user experience.

backend/templates/pages/admin.html

@@ -3,13 +3,19 @@
 {{define "content"}}
 <section class="app-shell admin-shell" aria-labelledby="admin-title">
   <aside class="app-sidebar">
-    <a class="sidebar-link {{if eq .Data.Section "overview"}}is-active{{end}}" href="/admin">Overview</a>
-    <a class="sidebar-link {{if eq .Data.Section "files"}}is-active{{end}}" href="/admin/files">Files</a>
-    <a class="sidebar-link" href="/admin/users">Users</a>
-    <a class="sidebar-link" href="/admin/settings">Settings</a>
-    <a class="sidebar-link" href="/app">My files</a>
+    <nav class="sidebar-nav">
+      <a class="sidebar-link {{if eq .Data.Section "overview"}}is-active{{end}}" href="/admin">Overview</a>
+      <a class="sidebar-link {{if eq .Data.Section "files"}}is-active{{end}}" href="/admin/files">Files</a>
+      <a class="sidebar-link" href="/admin/users">Users</a>
+      <a class="sidebar-link" href="/admin/settings">Settings</a>
+    </nav>
+    <hr class="sidebar-sep">
+    <nav class="sidebar-nav">
+      <a class="sidebar-link" href="/app">My Files</a>
+    </nav>
+    <hr class="sidebar-sep">
     <form class="sidebar-logout" action="/admin/logout" method="post">
-      <button class="button button-outline" type="submit">Logout</button>
+      <button class="button button-outline" type="submit">Sign out</button>
     </form>
   </aside>