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/src/lib/observe.ts

@@ -0,0 +1,66 @@
+// EAS Observe wrapper. Keeps expo-observe usage in one place and fails soft
+// so a metrics problem never blocks app startup or crash reporting.
+import { Observe } from "expo-observe";
+
+type ErrorHandler = (error: unknown, isFatal: boolean) => void;
+
+interface GlobalWithErrorUtils {
+  ErrorUtils?: {
+    getGlobalHandler(): ErrorHandler;
+    setGlobalHandler(handler: ErrorHandler): void;
+  };
+}
+
+let appStartedLogged = false;
+
+/**
+ * Logs the `app_started` custom event once per process lifetime. Safe to call
+ * more than once; only the first call has an effect.
+ */
+export function logAppStarted() {
+  if (appStartedLogged) return;
+  appStartedLogged = true;
+
+  try {
+    Observe.logEvent("app_started");
+  } catch (error) {
+    console.warn("[Observe] Failed to log app_started:", error);
+  }
+}
+
+let globalErrorHandlerInstalled = false;
+
+/**
+ * Installs a global fatal JS error handler that records an `xprem_js_crash`
+ * Observe event and then always delegates to whatever handler was previously
+ * registered (RN's default redbox handler, or another handler such as
+ * Crashlytics's, if one is installed first). Existing crash reporting is
+ * never replaced, only observed.
+ */
+export function installObserveGlobalErrorHandler() {
+  if (globalErrorHandlerInstalled) return;
+
+  const errorUtils = (global as GlobalWithErrorUtils).ErrorUtils;
+  if (!errorUtils) {
+    console.warn("[Observe] ErrorUtils is unavailable; xprem_js_crash will not be recorded.");
+    return;
+  }
+
+  globalErrorHandlerInstalled = true;
+  const previousHandler = errorUtils.getGlobalHandler();
+
+  errorUtils.setGlobalHandler((error, isFatal) => {
+    try {
+      const message = error instanceof Error ? error.message : String(error);
+      Observe.logEvent("xprem_js_crash", {
+        severity: "fatal",
+        body: message,
+        attributes: { isFatal: Boolean(isFatal) },
+      });
+    } catch (observeError) {
+      console.warn("[Observe] Failed to log xprem_js_crash:", observeError);
+    }
+
+    previousHandler(error, isFatal);
+  });
+}