feat(mobile): add expo-observe update health tracking

Install expo-observe (56.0.29, matching the SDK 56 line) and configure
its endpoint under expo.extra.eas.observe in app.json, alongside the
existing EAS projectId. Leave dispatchInDebug and sampleRate at their
defaults.

Log an app_started custom event once per cold start from the root
layout's existing "run once" startup effect, and install a global JS
error handler that logs xprem_js_crash and then always delegates to
whatever handler was previously registered, so existing Crashlytics
reporting is unaffected.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/mobile/app/_layout.tsx

@@ -5,6 +5,11 @@ import "../src/polyfills/web";
 import { initCrashlytics, setUserContext, clearUserContext } from "../src/lib/crashlytics";
 initCrashlytics();
 
+// Install the Observe global JS error handler after Crashlytics so it wraps
+// (and always calls) whatever handler is already registered.
+import { installObserveGlobalErrorHandler, logAppStarted } from "../src/lib/observe";
+installObserveGlobalErrorHandler();
+
 // Analytics imports (initialization deferred until app mounts)
 import { initAnalytics, setUserId, clearUserId, logScreenView, Events } from "../src/lib/analytics";
 
@@ -73,6 +78,7 @@ export default function RootLayout() {
             analyticsInitialized.current = true;
             initAnalytics();
             Events.appOpened("cold");
+            logAppStarted();
         }
     }, []);
 
@@ -115,15 +121,15 @@ export default function RootLayout() {
         // Fetch user on mount
         fetchUser();
 
-        // Handle initial URL if app was opened via deep link
-        const handleDeepLink = async (url: string | null) => {
-            if (!url) return;
+        // Handle initial URL if app was opened via deep link
+        const handleDeepLink = async (url: string | null) => {
+            if (!url) return;
 
             // Invite links (https://sauci.app/join/{code} or app.sauci://join?code={code})
             // are handled separately from the Supabase auth deep link flow below.
-            const inviteLink = parseInviteLinkCode(url);
-            if (inviteLink) {
-                Events.inviteLinkOpened(inviteLink.source);
+            const inviteLink = parseInviteLinkCode(url);
+            if (inviteLink) {
+                Events.inviteLinkOpened(inviteLink.source);
                 await stashPendingInviteCode(inviteLink.code);
 
                 const { isAuthenticated, user } = useAuthStore.getState();
@@ -200,15 +206,15 @@ export default function RootLayout() {
             }
         };
 
-        // Get initial URL
-        Linking.getInitialURL().then((url) => {
-            handleDeepLink(url);
-        });
+        // Get initial URL
+        Linking.getInitialURL().then((url) => {
+            handleDeepLink(url);
+        });
 
-        // Listen for new deep links while app is open
-        const linkingSubscription = Linking.addEventListener("url", (event) => {
-            handleDeepLink(event.url);
-        });
+        // Listen for new deep links while app is open
+        const linkingSubscription = Linking.addEventListener("url", (event) => {
+            handleDeepLink(event.url);
+        });
 
         // Set up push notification response listener (for taps while app is running)
         const notificationSubscription = setupNotificationResponseListener();