fix(onboarding): clarify opencode-serve requirement + fail connect fast (retention) (#107)

* fix(onboarding): clarify opencode-serve requirement and fail connect tests fast

New users bounce at ~0% 7-day retention because nothing tells them the app
needs a computer running `opencode serve` on the same network/Tailscale, and
a bad IP hangs for the full 30s request timeout before failing.

- Rewrite the no-connection empty state subtitle and add a "How to set up a
  server" link to the setup guide (app/(tabs)/index.tsx, src/lib/links.ts).
- Surface the opencode-serve prerequisite as a one-line notice at the top of
  the Quick Connect form, above the existing detailed help box
  (app/connection/add.tsx).
- Give the interactive connection test (testConnection) its own 12s timeout
  via an optional Client.global.health(timeoutMs) parameter, instead of
  reusing the general 30s REQUEST_TIMEOUT_MS used for real session traffic
  (src/lib/sdk.ts, src/stores/connections.ts).
- Mirror all new/changed strings in the zh-Hans catalog; catalog-parity test
  keeps them in sync.

* docs(distribution): add retention analysis motivating first-run fixes

Diagnoses ~0% D7 retention as product-shape (no path to value without a
self-hosted server, no demo mode, store copy sets no expectation). Ranks
fixes and isolates the two owner-only strategic calls (store-copy honesty,
hosted OpenCode Connect).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T12AhSnQVrSxNnvwfCx2z6

* fix(onboarding): drop connect-screen prerequisite notice (kept off-screen the submit button in E2E)

The added notice pushed connect-submit-button below the fold, breaking the
Maestro activation-positive flow (and the other flows sharing the connect
prelude). The empty state already sets the opencode-serve expectation one
screen earlier, so this notice was redundant. Empty-state guidance + guide
link and the fast-fail connect timeout are unaffected and retained.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T12AhSnQVrSxNnvwfCx2z6

---------

Co-authored-by: engineer <engineer@macbookpro.lan>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

app/(tabs)/index.tsx

@@ -14,6 +14,7 @@ import {
   Alert,
   KeyboardAvoidingView,
   Platform,
+  Linking,
 } from "react-native"
 import { router, useFocusEffect } from "expo-router"
 import { Ionicons } from "@expo/vector-icons"
@@ -27,6 +28,7 @@ import type { Session, Project } from "../../src/lib/sdk"
 import { DirectorySwitcher, DirectoryBrowserSheet } from "../../src/components/chat"
 import { groupByDirectory } from "../../src/lib/session-grouping"
 import { nameOf } from "../../src/lib/path-utils"
+import { SETUP_GUIDE_URL } from "../../src/lib/links"
 
 function formatTime(timestamp: number, t: (key: string, opts?: Record<string, unknown>) => string): string {
   const date = new Date(timestamp)
@@ -435,6 +437,13 @@ export default function SessionsScreen() {
             {t("sessionsList.empty.addConnectionButton")}
           </Text>
         </TouchableOpacity>
+        <TouchableOpacity
+          style={styles.setupGuideLink}
+          onPress={() => Linking.openURL(SETUP_GUIDE_URL)}
+          testID="setup-guide-link"
+        >
+          <Text style={styles.setupGuideLinkText}>{t("sessionsList.empty.setupGuideLink")}</Text>
+        </TouchableOpacity>
       </View>
     )
   }
@@ -1013,6 +1022,14 @@ const styles = StyleSheet.create({
   addButtonTextDark: {
     color: "#0a0a0a",
   },
+  setupGuideLink: {
+    marginTop: 16,
+  },
+  setupGuideLinkText: {
+    fontSize: 14,
+    fontWeight: "600",
+    color: "#6366f1",
+  },
   loadingContainer: {
     flex: 1,
     justifyContent: "center",