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/import.php

@@ -345,10 +345,10 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_FILES['csv_file'])) {
             <label class="admin-label">Fichier CSV :</label>
             <div class="admin-file-input">
                 <input type="file" id="csv_file" name="csv_file" accept=".csv" required>
-                <div class="admin-hint" style="margin-top:.5rem;">
+                <small style="margin-top:.5rem;">
                     Colonnes attendues : Identifiant, Titre, Sous-titre, Auteur·ice(s), Contact, Promoteur·ice(s), Format, Année, AP, Orientation, Finalité, Mots-clés, Synopsis, Contexte, Remarques, Langue, Autorisation, License, taille, Points sur 20, lien BAIU<br>
                     — Deux premières lignes ignorées (en-tête) — Séparateur : virgule — Encodage : UTF-8
-                </div>
+                </small>
             </div>
         </div>