launcher.php: fix PHP 8 fatal in saveManifest

`usort($manifest['versions'] ?? [], ...)` worked in PHP 7 but is a
fatal error in PHP 8 — usort requires its first arg by reference and
the null-coalesce operator produces an rvalue, not a variable. The
existing versions.php didn't trigger it because it sorts
$manifest['versions'] directly. Fixed launcher.php to guard with an
isset/is_array check and only sort when there's actually a versions
array.

Triggered by clicking "Définir" in the new Launcher page → 500 page.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/admin/launcher.php

@@ -23,7 +23,10 @@ function loadManifest(string $path): array
 
 function saveManifest(string $path, array $manifest): void
 {
-    usort($manifest['versions'] ?? [], fn($a, $b) => version_compare($b['version'], $a['version']));
+    // usort attend une référence — on ne peut pas lui passer `?? []` en PHP 8.
+    if (isset($manifest['versions']) && is_array($manifest['versions'])) {
+        usort($manifest['versions'], fn($a, $b) => version_compare($b['version'], $a['version']));
+    }
     file_put_contents(
         $path,
         json_encode($manifest, JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES) . "\n"