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/edit.php

@@ -19,10 +19,7 @@ if ($thesisId <= 0) {
     die("ID invalide");
 }
 
-// Consume flash messages from the edit action
-$error   = $_SESSION['edit_error']   ?? null;
-$success = $_SESSION['edit_success'] ?? null;
-unset($_SESSION['edit_error'], $_SESSION['edit_success']);
+// Flash messages are consumed by the flash-messages partial below.
 
 try {
     $db = new Database();
@@ -67,12 +64,7 @@ try {
 <main id="main-content">
     <h1>Modifier un TFE</h1>
 
-    <?php if ($error): ?>
-    <p role="alert" data-type="error">⚠ <?= htmlspecialchars($error) ?></p>
-    <?php endif; ?>
-    <?php if ($success): ?>
-    <p role="status" data-type="success">✓ <?= htmlspecialchars($success) ?></p>
-    <?php endif; ?>
+    <?php include APP_ROOT . '/templates/partials/flash-messages.php'; ?>
 
     <form method="post" action="/admin/actions/edit.php" class="admin-form" enctype="multipart/form-data">
         <input type="hidden" name="csrf_token" value="<?= htmlspecialchars($_SESSION['csrf_token']) ?>">