feat: v6.0.0 PWA offline support

- manifest + icones, service worker (precache, network-first, Background Sync)

- module client FlowOffline (IndexedDB, queue, delta, flush) + hook editeur

- endpoints /api/v2/sync/{delta,batch,status} + moteur de sync (conflits LWW/orpheline/copie offline)

- migrations offline_sync_queue + sync_version (triggers)

- UI offline (banner, badge sync, toasts, icone dirty) + doc /help

- tests pytest (sync, migrations, SW, offline) + E2E Playwright; bump 6.0.0

e2e/package.json

@@ -4,7 +4,8 @@
   "description": "",
   "main": "index.js",
   "scripts": {
-    "test": "echo \"Error: no test specified\" && exit 1"
+    "test": "playwright test",
+    "test:pwa": "playwright test pwa_offline.spec.js"
   },
   "keywords": [],
   "author": "",