feat: production-ready UI with onboarding wizard and dashboard

- Onboarding flow (welcome -> Codex setup -> Claude setup -> done) with
  dark-themed screens and step indicators; completion stored in SecureStore
- Smart root redirect checks onboarding state on launch
- Dashboard tab shows both services at a glance via ServiceStatusCard;
  useFocusEffect + reload() keeps it fresh when credentials change in tabs
- Settings tab manages credentials and exposes re-run setup wizard
- Polished Codex and Claude detail screens with service headers and empty states
- 4-tab layout (Home, Codex, Claude, Settings) with dark-mode tab bar
- Both hooks gain reload() for cross-tab credential refresh

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/onboarding/_layout.tsx

@@ -0,0 +1,12 @@
+import { Stack } from "expo-router";
+
+export default function OnboardingLayout() {
+  return (
+    <Stack
+      screenOptions={{
+        headerShown: false,
+        animation: "slide_from_right",
+      }}
+    />
+  );
+}