Unify flash messages: replace all legacy session key writes with App::flash()

All admin action files (account, tag, page, edit, visibility, maintenance,
publish, formulaire) now call App::flash('error'|'success', ...) instead of
writing to raw per-page session keys ($_SESSION['error'], 'admin_error',
'edit_error', 'admin_success', 'edit_success', 'form_error').

All admin display pages (add, edit, account, tags, pages, index) now include
templates/partials/flash-messages.php instead of manually reading and
unsetting the legacy session keys and inlining their own alert HTML.

App::consumeFlash() already drained all legacy key variants as a safety net,
so the partial works correctly whether called from pages that were already
migrated or any remaining stragglers. No behaviour change for end users.

public/admin/actions/formulaire.php

@@ -312,7 +312,7 @@ try {
     error_log("Form processing error: " . $e->getMessage());
 
     // Save error message and form data to session
-    $_SESSION['form_error'] = $e->getMessage();
+    App::flash('error', $e->getMessage());
     $_SESSION['form_data'] = $_POST;
 
     // Redirect back to form with preserved data