Extract shared TFE form partial — single source of truth for add/edit/partage

Created templates/partials/form/form.php as the unified form template driven by
$mode ('add'|'edit'|'partage') and boolean flags for optional sections.

The three calling templates (templates/admin/add.php, templates/admin/edit.php,
partage/index.php renderShareLinkForm) now only set variables then include the
shared partial. ~200 lines of duplicated fieldset HTML eliminated.

app/templates/partials/form/fieldset-tfe-info.php

@@ -71,4 +71,4 @@ $synopsisExtra   = $synopsisExtra   ?? '';
     </div>
 </fieldset>
 <?php
-unset($oldFn, $withAutofocusFn, $allowedObjet, $synopsisExtra);
+unset($allowedObjet, $synopsisExtra);