test: I2 + I3 — 46 tests d'intégration + suite E2E Playwright

I2 (test_pane_manager.py): 46 nouveaux tests
  - Module structure: 10 tests (exports, methods, factory, max panes)
  - Data flow: 8 tests (drag format, tree format, JSON, migration, localStorage)
  - DOM flow: 7 tests (grid, IDs, lazy render, collapse, empty pane, transfer)
  - Keyboard: 6 tests (all shortcuts + preventDefault counts)
  - Context menu: 4 tests (split, move-to, close actions)
  - CSS: 7 tests (grid, container, resize, drop zone, buttons, responsive)
  - HTML/App: 5 tests (wrapper, tab-bar, modules, imports)
  - Palette: 3 tests (commands, reset, category)
  - Editor lock: 2 tests
  - Dashboard: 1 test (TabManager vs openFile)

I3 (tests/e2e/): suite Playwright complète
  - package.json, playwright.config.js, split-view.spec.js
  - 12 test specs: basic ops, tab drag, tree drag, persistence, regression, keyboard nav
  - Exécution: cd tests/e2e && npm install && npx playwright test

Total: 353 tests passent (307 + 46)

tests/e2e/package.json

@@ -0,0 +1,11 @@
+{
+  "name": "obsigate-e2e",
+  "private": true,
+  "devDependencies": {
+    "@playwright/test": "^1.45.0"
+  },
+  "scripts": {
+    "test": "npx playwright test",
+    "test:headed": "npx playwright test --headed"
+  }
+}