fix(app): reduce spurious SSE reconnects after sleep/resume

Two changes to improve connection stability when a device resumes from
sleep or a browser tab returns from background:

1. Increase HEARTBEAT_TIMEOUT_MS from 15s to 30s. The server sends
   heartbeats every 10s, so 15s left only 5s of margin — under load or
   network jitter a heartbeat could arrive late and trigger a premature
   disconnect. 30s provides a comfortable 20s buffer while still
   detecting genuinely dead connections.

2. On visibilitychange, reset the heartbeat timer instead of immediately
   aborting. Previously, returning to a backgrounded tab after >15s would
   instantly abort the SSE connection and trigger a full reconnect cycle
   (re-bootstrap all directories). Now the stream gets one full heartbeat
   interval to prove it's still alive. If no event arrives within the
   timeout, the timer fires and aborts — same end result but without
   false positives for connections that are still healthy.

Author: BYK

packages/app/src/context/global-sdk.tsx

@@ -105,7 +105,11 @@ export const { use: useGlobalSDK, provider: GlobalSDKProvider } = createSimpleCo
     const aborted = (error: unknown) => abortError.safeParse(error).success
 
     let attempt: AbortController | undefined
-    const HEARTBEAT_TIMEOUT_MS = 15_000
+    // Must exceed the server's heartbeat interval (10s) with margin for
+    // network jitter and event-loop delays. Too tight causes spurious
+    // reconnects during idle periods; too loose delays detecting a dead
+    // connection after sleep/resume.
+    const HEARTBEAT_TIMEOUT_MS = 30_000
     let lastEventAt = Date.now()
     let heartbeat: ReturnType<typeof setTimeout> | undefined
     const resetHeartbeat = () => {
@@ -190,11 +194,16 @@ export const { use: useGlobalSDK, provider: GlobalSDKProvider } = createSimpleCo
       }
     })().finally(flush)
 
+    // When the tab becomes visible after being backgrounded, check if the
+    // connection is likely stale. Instead of aborting immediately (which
+    // tears down the stream even if it's still alive), restart the
+    // heartbeat timer so the stream gets one full interval to deliver an
+    // event. If nothing arrives within the timeout the timer will abort.
     const onVisibility = () => {
       if (typeof document === "undefined") return
       if (document.visibilityState !== "visible") return
       if (Date.now() - lastEventAt < HEARTBEAT_TIMEOUT_MS) return
-      attempt?.abort()
+      resetHeartbeat()
     }
     if (typeof document !== "undefined") {
       document.addEventListener("visibilitychange", onVisibility)