ver. 0.307: check-update button + auto-generated changelog

- Add "Sprawdź aktualizacje" refresh button in admin sidebar (AJAX check without page reload)
- Add UpdateController::checkUpdate() action clearing session cache and querying update server
- Replace hand-edited changelog.php with auto-generating script (reads manifests + legacy JSON)
- Migrate all legacy changelog entries (0.300-0.001) to changelog-legacy.json

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

autoload/admin/Controllers/UpdateController.php

@@ -49,4 +49,17 @@ class UpdateController
         echo json_encode( $response );
         exit;
     }
+
+    public function checkUpdate(): void
+    {
+        \Shared\Helpers\Helpers::set_session( 'new-version', null );
+        $newVer = \Shared\Helpers\Helpers::get_new_version();
+        $curVer = \Shared\Helpers\Helpers::get_version();
+
+        echo json_encode( [
+            'has_update' => $newVer > $curVer,
+            'new_ver'    => $newVer,
+        ] );
+        exit;
+    }
 }