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/pages.php

@@ -15,8 +15,7 @@ try {
     die("Erreur lors du chargement des pages.");
 }
 
-$success = $_SESSION['success'] ?? null;
-unset($_SESSION['success']);
+// Flash messages are consumed by the flash-messages partial below.
 ?>
 <?php $isAdmin = true; $bodyClass = 'admin-body'; require_once APP_ROOT . '/templates/head.php'; ?>
     <?php include APP_ROOT . '/templates/header.php'; ?>
@@ -24,9 +23,7 @@ unset($_SESSION['success']);
 <main id="main-content">
     <h1>Pages statiques</h1>
 
-    <?php if ($success): ?>
-    <p role="status" data-type="success">✓ <?= htmlspecialchars($success) ?></p>
-    <?php endif; ?>
+    <?php include APP_ROOT . '/templates/partials/flash-messages.php'; ?>
 
     <table>
         <thead>