Add field-level aria-errormessage, aria-invalid, and aria-describedby across the TFE form

WCAG 3.3.1 (Error Identification): failing fields now get
aria-errormessage pointing to the flash-error container and
aria-invalid="true". WCAG 3.3.3 (Error Suggestion): <small>
hint text on inputs, selects, and file fields is now linked via
aria-describedby (always, not just on error).

Changes:
- text-field.php, select-field.php, checkbox-list.php: accept
  $errorFieldName; add aria-errormessage/aria-invalid on match;
  add id to <small> and aria-describedby on the control
- fieldset-tfe-info.php: aria-invalid on synopsis textarea
- fichiers-fragment.php: aria-describedby on cover, note
  d'intention, TFE, annexes, and website inputs; aria-invalid
  on format checkboxes when error matches 'formats'
- form.php: id="flash-error" + tabindex="-1" on flash-error
  div; accept $errorFieldName from callers
- admin/add.php: set $errorFieldName, wire $withAutofocusFn
  (was identity default)
- admin/edit.php: set $errorFieldName
- partage/index.php: consume autofocus field, wire autofocus
  function, add App::flashAutofocus() in submit catch block

Also fixes WCAG standards issue: removed invalid 'required'
HTML attribute from <fieldset> elements in checkbox-list.php
and fichiers-fragment.php (only aria-required stays). Added
role="group" for explicit ARIA semantics.

app/templates/partials/form/select-field.php

@@ -14,6 +14,7 @@
  *                           set to null to suppress the empty option entirely
  *   string|null $id       — override the id attribute (defaults to $name)
  *   string|null $hint     — optional hint shown in <small> below the select
+ *   string|null $errorFieldName — when set and matches $name, adds aria-invalid + aria-errormessage
  */
 
 $required    = $required    ?? false;
@@ -21,6 +22,18 @@ $placeholder = array_key_exists('placeholder', get_defined_vars()) ? $placeholde
 $id          = $id          ?? $name;
 $hint        = $hint        ?? null;
 $attrs       = $attrs       ?? [];
+$errorFieldName = $errorFieldName ?? null;
+
+// Build hint id for aria-describedby
+$hintId = ($hint !== null) ? ($id . '-hint') : null;
+
+// Build describedby string (hint + error)
+$describedBy = [];
+if ($hintId !== null) $describedBy[] = $hintId;
+if ($errorFieldName === $name) $describedBy[] = 'flash-error';
+$ariaDescribedBy = !empty($describedBy) ? ' aria-describedby="' . implode(' ', $describedBy) . '"' : '';
+
+$ariaInvalid = ($errorFieldName === $name) ? ' aria-invalid="true" aria-errormessage="flash-error"' : '';
 ?>
 <div>
     <label for="<?= htmlspecialchars($id) ?>"><?= htmlspecialchars($label) ?><?= $required ? ' <span class="asterisk">*</span>' : '' ?></label>
@@ -37,7 +50,9 @@ foreach ($attrs as $k => $v) {
     <select id="<?= htmlspecialchars($id) ?>"
             name="<?= htmlspecialchars($name) ?>"
             <?= $required ? 'required' : '' ?>
-            <?= $selectAttrStr ?>>
+            <?= $selectAttrStr ?>
+            <?= $ariaInvalid ?>
+            <?= $ariaDescribedBy ?>>
         <?php if ($placeholder !== null): ?>
         <option value=""><?= htmlspecialchars($placeholder) ?></option>
         <?php endif; ?>
@@ -54,8 +69,8 @@ foreach ($attrs as $k => $v) {
         <?php endforeach; ?>
     </select>
     <?php if ($hint): ?>
-    <small><?= htmlspecialchars($hint) ?></small>
+    <small id="<?= htmlspecialchars($hintId) ?>"><?= htmlspecialchars($hint) ?></small>
     <?php endif; ?>
 </div>
 <?php
-unset($required, $placeholder, $id, $hint, $attrs, $selectAttrStr, $k, $v);
+unset($required, $placeholder, $id, $hint, $attrs, $selectAttrStr, $k, $v, $hintId, $describedBy, $ariaDescribedBy, $ariaInvalid, $errorFieldName);