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>

app/demo.tsx

@@ -0,0 +1,124 @@
+import { useCallback, useMemo, useState } from "react"
+import { View, Text, ScrollView, TouchableOpacity, StyleSheet, useColorScheme, Linking } from "react-native"
+import { Stack, useRouter } from "expo-router"
+import { Ionicons } from "@expo/vector-icons"
+import { useTranslation } from "react-i18next"
+import { MessageBubble, PermissionPrompt } from "../src/components/chat"
+import { buildDemoScript, buildDemoCompletionMessage, buildDemoDenialMessage } from "../src/lib/demo-script"
+import { SETUP_GUIDE_URL } from "../src/lib/links"
+
+// Fully offline, scripted walkthrough for installers with no self-hosted
+// opencode server. ISOLATION: every message/part below is hardcoded local
+// data built once with useMemo — this screen never calls useSessions,
+// useConnections, useEvents, or any sdk.ts client method, so it cannot
+// create a real session, touch the network, or corrupt real app state. The
+// permission "reply" only flips local component state (below); it never
+// calls sessionClient.permission.reply the way app/session/[id].tsx does.
+export default function DemoScreen() {
+  const router = useRouter()
+  const colorScheme = useColorScheme()
+  const isDark = colorScheme === "dark"
+  const { t } = useTranslation()
+
+  // Built once per mount from pure, hardcoded data (src/lib/demo-script.ts).
+  const script = useMemo(() => buildDemoScript(), [])
+  const [reply, setReply] = useState<"once" | "always" | "reject" | null>(null)
+
+  const handleReply = useCallback((r: "once" | "always" | "reject") => {
+    setReply(r)
+  }, [])
+
+  const completion = useMemo(() => {
+    if (!reply) return null
+    return reply === "reject" ? buildDemoDenialMessage() : buildDemoCompletionMessage()
+  }, [reply])
+
+  const [userMessage, assistantMessage] = script.messages
+
+  return (
+    <>
+      <Stack.Screen options={{ title: t("demo.title"), presentation: "card" }} />
+      <View style={[s.container, isDark && s.containerDark]} testID="demo-screen">
+        <View style={[s.banner, isDark && s.bannerDark]} testID="demo-banner">
+          <Ionicons name="play-circle-outline" size={16} color="#8b5cf6" />
+          <Text style={s.bannerText}>{t("demo.banner")}</Text>
+        </View>
+
+        <ScrollView contentContainerStyle={s.scrollContent} keyboardShouldPersistTaps="handled">
+          <MessageBubble message={userMessage} parts={script.parts[userMessage.id] || []} isDark={isDark} />
+          <MessageBubble message={assistantMessage} parts={script.parts[assistantMessage.id] || []} isDark={isDark} />
+
+          {!reply && <PermissionPrompt permission={script.permission} isDark={isDark} onReply={handleReply} />}
+
+          {completion && (
+            <MessageBubble message={completion.message} parts={completion.parts} isDark={isDark} />
+          )}
+
+          <View style={[s.ctaCard, isDark && s.ctaCardDark]} testID="demo-cta-card">
+            <Text style={[s.ctaTitle, isDark && s.textWhite]}>{t("demo.ctaTitle")}</Text>
+            <Text style={[s.ctaSubtitle, isDark && s.metaDark]}>{t("demo.ctaSubtitle")}</Text>
+            <TouchableOpacity
+              style={s.connectButton}
+              onPress={() => router.push("/connection/add")}
+              testID="demo-connect-button"
+            >
+              <Text style={s.connectButtonText}>{t("demo.connectButton")}</Text>
+            </TouchableOpacity>
+            <TouchableOpacity
+              style={s.setupGuideLink}
+              onPress={() => Linking.openURL(SETUP_GUIDE_URL)}
+              testID="demo-setup-guide-link"
+            >
+              <Text style={s.setupGuideLinkText}>{t("demo.setupGuideLink")}</Text>
+            </TouchableOpacity>
+          </View>
+        </ScrollView>
+      </View>
+    </>
+  )
+}
+
+const s = StyleSheet.create({
+  container: { flex: 1, backgroundColor: "#ffffff" },
+  containerDark: { backgroundColor: "#0a0a0a" },
+  textWhite: { color: "#ffffff" },
+  metaDark: { color: "#888888" },
+
+  banner: {
+    flexDirection: "row",
+    alignItems: "center",
+    gap: 8,
+    paddingHorizontal: 16,
+    paddingVertical: 10,
+    backgroundColor: "#f5f3ff",
+    borderBottomWidth: 1,
+    borderBottomColor: "#e9d5ff",
+  },
+  bannerDark: { backgroundColor: "#1a1030", borderBottomColor: "#2a1a4a" },
+  bannerText: { fontSize: 13, fontWeight: "600", color: "#6d28d9", flex: 1 },
+
+  scrollContent: { padding: 16, paddingBottom: 40 },
+
+  ctaCard: {
+    marginTop: 8,
+    padding: 20,
+    borderRadius: 16,
+    backgroundColor: "#f5f5f5",
+    alignItems: "center",
+  },
+  ctaCardDark: { backgroundColor: "#1a1a1a" },
+  ctaTitle: { fontSize: 17, fontWeight: "700", color: "#0a0a0a", textAlign: "center" },
+  ctaSubtitle: { fontSize: 13, color: "#666666", marginTop: 6, textAlign: "center", lineHeight: 18 },
+  connectButton: {
+    marginTop: 16,
+    backgroundColor: "#0a0a0a",
+    paddingHorizontal: 24,
+    paddingVertical: 12,
+    borderRadius: 8,
+    width: "100%",
+    alignItems: "center",
+  },
+  connectButtonText: { color: "#ffffff", fontWeight: "600", fontSize: 15 },
+  setupGuideLink: { marginTop: 14 },
+  setupGuideLinkText: { fontSize: 14, fontWeight: "600", color: "#6366f1" },
+})