feat(demo): add offline demo mode for zero-server activation (#108)

Installers with no self-hosted opencode server hit a dead end at the
empty Sessions state, contributing to ~0% 7-day retention. Adds a
fully offline, scripted /demo route reusing the real chat components
(MessageBubble, ToolCallCard/DiffView, PermissionPrompt) so new users
can see what opencode does before connecting anything, then funnels
them to Connect / the setup guide.

- src/lib/demo-script.ts: pure, hardcoded Message/Part fixture builder
  (no RN/store/network imports) — the isolation guarantee.
- app/demo.tsx: new /demo route rendering the scripted conversation
  via useMemo'd local state only; permission reply is local setState,
  never sessionClient.permission.reply().
- app/(tabs)/index.tsx: "Try a demo" button added to the no-connection
  empty state, placed after the existing add-connection-button so its
  position/testID for existing Maestro flows is unchanged.
- .maestro/flows/demo.yaml: new E2E flow covering the empty-state CTA
  through conversation, diff expand, permission approve, and the CTA
  reaching the real connect form.
- scripts/run-e2e-flows.sh: registers demo in NEWER_FLOWS (non-blocking)
  so it actually runs in CI.
- i18n: new sessionsList.empty.tryDemoButton and demo.* keys added to
  both en.json and zh-Hans.json (catalog-parity verified).

npm run typecheck: clean. npm test: 175/175 passing.

Co-authored-by: engineer <engineer@macbookpro.lan>

src/lib/i18n/en.json

@@ -259,6 +259,7 @@
       "noConnectionSubtitle": "Connect to a computer running opencode serve on your network or via Tailscale to get started.",
       "setupGuideLink": "How to set up a server",
       "addConnectionButton": "Add Connection",
+      "tryDemoButton": "Try a demo (no server needed)",
       "authFailedTitle": "Authentication Failed",
       "authFailedSubtitle": "{{name}} rejected your credentials. Check the username and password to reconnect.",
       "checkCredentialsButton": "Check Credentials",
@@ -424,5 +425,13 @@
         "daysAgo": "{{count}}d ago"
       }
     }
+  },
+  "demo": {
+    "title": "Demo",
+    "banner": "Demo — sample conversation, no server connected",
+    "ctaTitle": "Like what you see?",
+    "ctaSubtitle": "This was a scripted demo. Connect your own opencode server to do this for real.",
+    "connectButton": "Connect your own server",
+    "setupGuideLink": "How to set up a server"
   }
 }