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/fieldset-tfe-info.php

@@ -19,6 +19,7 @@ $allowedObjet    = $allowedObjet    ?? [];
 $formData        = $formData        ?? [];
 $synopsisExtra   = $synopsisExtra   ?? '';
 $adminMode       = $adminMode       ?? false;
+$errorFieldName  = $errorFieldName  ?? null;
 ?>
 <fieldset>
     <legend>Informations du TFE</legend>
@@ -72,7 +73,8 @@ $adminMode       = $adminMode       ?? false;
         <label for="synopsis">Synopsis :<?= $adminMode ? '' : ' <span class="asterisk">*</span>' ?></label>
         <?= $synopsisExtra ?>
         <textarea id="synopsis" name="synopsis" rows="7" <?= $adminMode ? '' : 'required' ?>
-                  <?= ($withAutofocusFn('synopsis')['autofocus'] ?? false) ? 'autofocus' : '' ?>><?= $oldFn('synopsis') ?></textarea>
+                  <?= ($withAutofocusFn('synopsis')['autofocus'] ?? false) ? 'autofocus' : '' ?>
+                  <?= ($errorFieldName === 'synopsis') ? 'aria-invalid="true" aria-errormessage="flash-error" aria-describedby="flash-error"' : '' ?>><?= $oldFn('synopsis') ?></textarea>
     </div>
 </fieldset>
 <?php