Replace .admin-hint / .admin-field-hint with .admin-body form small

- admin.css: remove .admin-hint and .admin-field-hint class rules; add
  .admin-body form small with the same font-size/color/margin properties
  plus display:block so it stacks below sibling inputs; stub comment left
  where .admin-field-hint was to document the change
- add.php: 5× <p class="admin-hint"> → <small>
- edit.php: 3× <p class="admin-hint"> → <small>
- import.php: <div class="admin-hint"> → <small> (block hint below CSV input)
- pages-edit.php: class="admin-hint" removed from already-correct <small>
- account.php: <p class="admin-field-hint"> → <small>

Hint text is now styled purely via the semantic element selector; no class
required on any hint element in admin templates.

public/admin/account.php

@@ -78,7 +78,7 @@ if (empty($_SESSION['csrf_token'])) {
                 <input class="admin-input" type="password" id="new_password"
                        name="new_password" required autocomplete="new-password"
                        minlength="12">
-                <p class="admin-field-hint">Minimum 12 caractères.</p>
+                <small>Minimum 12 caractères.</small>
             </div>
         </div>