P4-C: PWA installable, service worker push & campagne de fiabilité dialogues

- PWA: public/manifest.webmanifest + icon.svg + meta index.html (theme-color, apple-touch-icon)
- public/sw.js: service worker push-only (push → showNotification tag=sessionId ;
  notificationclick → focus/ouvre /sessions/<id>) — pas de précache (hors périmètre)
- lib/push.ts + stores/push.ts: enable/disable, abonnement VAPID, enregistrement SW au boot
- DashboardView: toggle « Notifications » (gardé par pushSupported) + i18n en/fr
- campagne de fiabilité (verdict S3): classification de tous les types de dialogue sur captures
  réelles (trust/permission×2/question) + refus Esc (deny-esc2) + plan (synthétique)
- acceptance-p4.mjs: VAPID/auth/Origin, subscribe idempotent/unsubscribe, answer (rejets)

Choix: SW écrit à la main plutôt que vite-plugin-pwa (qui tirait ~295 paquets Workbox +
2 vulns esbuild high pour un SW push-only). Zéro dépendance front nouvelle. 175 tests verts,
acceptance p1..p4 ALL GREEN.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/web/src/i18n/en.ts

@@ -97,6 +97,12 @@ export default {
     back: 'Sessions',
     notAttachable: 'This session runs outside Arboretum — fork it to interact, or resume it once it has stopped.',
   },
+  push: {
+    enable: 'Enable notifications',
+    disable: 'Notifications on',
+    denied: 'Notification permission denied — enable it in your browser settings.',
+    unsupported: 'Push needs HTTPS (e.g. Tailscale Serve); on iOS, install the app to your home screen first.',
+  },
   ws: {
     reconnecting: 'Connection lost — reconnecting…',
   },