test(pane-manager): tests JSDOM pour #75 I2

- tests/frontend/pane-manager.test.mjs : 9 tests d'intégration frontend via JSDOM 30
  Couvre factory, init, open/activate/close, isolation entre panes, drag/drop smoke
- tests/frontend/package.json + package-lock.json : jsdom en devDependency
- .gitea/workflows/ci.yml : nouvelle étape 'Frontend JSDOM tests (PaneManager)'
  qui installe jsdom au besoin puis lance pane-manager.test.mjs

tests/frontend/package.json

@@ -0,0 +1,16 @@
+{
+  "name": "frontend",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "type": "commonjs",
+  "devDependencies": {
+    "jsdom": "^30.0.1"
+  }
+}