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/index.php

@@ -67,12 +67,7 @@ document.addEventListener('DOMContentLoaded', () => {
 <main id="main-content">
     <h1>Liste des TFE</h1>
 
-    <?php if (isset($_SESSION['error'])): ?>
-    <p role="alert" data-type="error">⚠ <?= htmlspecialchars($_SESSION['error']); unset($_SESSION['error']); ?></p>
-    <?php endif; ?>
-    <?php if (isset($_SESSION['success'])): ?>
-    <p role="status" data-type="success">✓ <?= htmlspecialchars($_SESSION['success']); unset($_SESSION['success']); ?></p>
-    <?php endif; ?>
+    <?php include APP_ROOT . '/templates/partials/flash-messages.php'; ?>
 
     <!-- Maintenance mode toggle -->
     <?php $maintenanceOn = file_exists(APP_ROOT . '/storage/maintenance.flag'); ?>