refactor: rename "folder" to "package" throughout codebase

- Rename CSS files: _card-folder.scss → card-package
- Rename template files: _folder.scss → package
- Update all CSS class names and selectors
- Delete old folder blueprints and templates
- Add new package blueprints with improved structure
- Update investigation-summary template and blueprint
- Add "Dossiers" menu item in panel configuration
- Improve package blueprint with linkedContent field

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

site/config/menu.php

@@ -22,6 +22,15 @@ return [
   ],
   '-'
   ,
+  'dossiers' => [
+    'label' => 'Dossiers',
+    'icon' => 'folder',
+    'link' => 'pages/dossiers',
+    'current' => function (string $current): bool {
+        $path = Kirby\Cms\App::instance()->path();
+        return Str::contains($path, 'pages/dossiers');
+    }
+  ],
   'database' => [
     'label' => 'Bases de données',
     'icon' => 'table',