css: replace admin-form-row/admin-label/admin-input/select/textarea classes with semantic selectors

Remove five presentational classes from admin forms and replace with
structural CSS selectors scoped to .admin-form:

- .admin-form-row  → .admin-form > div:not(.admin-submit-wrap)
  Grid layout (260px label col + 1fr input col) applied directly to div
  children of the form; submit-wrap div excluded via :not().

- .admin-label     → .admin-form > div:not(.admin-submit-wrap) > label
  Scoped to the direct label child of each form row div; does not bleed
  into nested checkbox labels inside .admin-checkbox-list.

- .admin-input / .admin-select / .admin-textarea
  → .admin-form input:not([type=checkbox|radio|file|hidden|submit])
  → .admin-form select
  → .admin-form textarea
  Also extended to .admin-inline-form input/select (tags page) so the
  tags table inputs retain identical base styling and focus colour.

Templates updated: add.php, edit.php, login.php, account.php,
pages-edit.php, import.php, tags.php,
templates/partials/form/jury-fieldset.php — all class= attributes for
the five removed classes stripped.

import.php: added 'admin-form' class alongside 'admin-import-area' so
its single file-input row gets the grid row treatment; submit div was
already using admin-submit-wrap so it is correctly excluded.

No visual change — selectors target the same elements as before.

public/admin/pages-edit.php

@@ -88,10 +88,10 @@ JS;
         <input type="hidden" name="csrf_token" value="<?= htmlspecialchars($_SESSION['csrf_token']) ?>">
         <input type="hidden" name="slug" value="<?= htmlspecialchars($slug) ?>">
 
-        <div class="admin-form-row">
-            <label class="admin-label" for="content">Contenu (Markdown) :</label>
+        <div>
+            <label for="content">Contenu (Markdown) :</label>
             <div>
-                <textarea class="admin-textarea" id="content" name="content"
+                <textarea id="content" name="content"
                           rows="20"><?= htmlspecialchars($page['content'] ?? '') ?></textarea>
                 <small>Appuyez sur <kbd>Échap</kbd> pour quitter l'éditeur au clavier.</small>
             </div>