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/form.php

@@ -95,6 +95,9 @@ $filesMode               = $filesMode           ?? 'add';
 $existingWebsiteUrl      = $existingWebsiteUrl  ?? '';
 $existingWebsiteLabel    = $existingWebsiteLabel ?? '';
 $checkedFormatsForSiteWeb = $checkedFormatsForSiteWeb ?? [];
+
+// WCAG 3.3.1: which field has a validation error (set by caller from App::consumeAutofocus())
+$errorFieldName = $errorFieldName ?? null;
 ?>
 
 <?php if ($showIntroHelp && isset($helpFn)): ?>
@@ -119,7 +122,7 @@ $checkedFormatsForSiteWeb = $checkedFormatsForSiteWeb ?? [];
     );
     ?>
     <?php if ($flashError): ?>
-    <div class="flash-error" role="alert"><?= htmlspecialchars(
+    <div class="flash-error" id="flash-error" role="alert" tabindex="-1"><?= htmlspecialchars(
         $flashError,
     ) ?></div>
     <?php endif; ?>