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/public/partage/index.php

@@ -338,10 +338,13 @@ function renderShareLinkForm(string $slug, array $link): void
     $pageTitle  = 'Soumettre un TFE';
     $isVerified = !empty($_SESSION['share_verified_' . $slug]);
 
+    // WCAG 3.3.1: which field has a validation error (set by submit handler via App::flashAutofocus)
+    $errorFieldName = App::consumeAutofocus();
+
     // Build old()-compatible callable from $formData (share forms use the array variant).
     $shareOldFn = fn(string $key, string $default = '') => old($formData, $key, $default);
-    // No autofocus in the share form - identity function.
-    $shareWithAutofocusFn = fn(string $field, array $attrs = []) => $attrs;
+    // Autofocus function for the share form
+    $shareWithAutofocusFn = fn(string $field, array $attrs = []) => ($errorFieldName === $field) ? ($attrs + ['autofocus' => true]) : $attrs;
 
     // Load all form help blocks in one query.
     $helpBlocks = Database::getInstance()->getAllFormHelpBlocks();
@@ -647,6 +650,12 @@ function handleShareLinkSubmission(string $slug): void
         storePrimedFiles($slug);
         $_SESSION[$shareCsrfKey] = bin2hex(random_bytes(32)); // Regenerate token
 
+        // WCAG 3.3.1: autofocus the failing field after redirect
+        $autofocusField = ThesisCreateController::autofocusFieldForError($e->getMessage());
+        if ($autofocusField !== null) {
+            App::flashAutofocus($autofocusField);
+        }
+
         // Redirect back to the form
         header('Location: /partage/' . urlencode($slug));
         exit();