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>

scripts/run-e2e-flows.sh

@@ -27,7 +27,9 @@ APK="android/app/build/outputs/apk/release/app-release.apk"
 # selectors/seeding surface) and moving each back into CORE once confirmed
 # green.
 CORE_FLOWS=(activation-positive activation-negative-401)
-NEWER_FLOWS=(directory-picker all-sessions variant-picker diff-scroll)
+# demo.yaml (offline "Try a demo" mode) needs no mock server — it never
+# connects — so it's safe to run alongside the others here unmodified.
+NEWER_FLOWS=(directory-picker all-sessions variant-picker diff-scroll demo)
 mkdir -p "$ROOT/artifacts/screenshots" "$ROOT/artifacts/diag"
 
 echo "== installing APK =="