Harden the update-check pipeline against silent failures (#58)

checkForUpdate() was a bare `void registration?.update()` -- a failed
fetch (most plausible right when it's triggered by a WS reconnect, i.e.
the network just flapped from a backend restart) vanished with nothing
caught or logged, leaving only the hourly interval as a fallback. Now
logs the failure instead of swallowing it, and a third trigger checks
for an update whenever a backgrounded tab becomes visible again, so a
tab that misses both the reconnect-triggered check and the hourly timer
still gets a chance the moment someone actually looks at it.

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

frontend/src/components/UpdateBanner.tsx

@@ -1,3 +1,4 @@
+import { useEffect } from 'react'
 import { useRegisterSW } from 'virtual:pwa-register/react'
 import { checkForUpdate, setSwRegistration } from '../lib/swUpdate'
 import './UpdateBanner.css'
@@ -26,6 +27,21 @@ export function UpdateBanner() {
     },
   })
 
+  useEffect(() => {
+    // #58: a third trigger, alongside WS-reconnect and the hourly interval
+    // above -- a tab backgrounded across a deploy gets checked the moment
+    // someone actually looks at it again, rather than waiting on whichever
+    // of those two happens to land first. Cheap insurance against either
+    // one missing its moment (e.g. the reconnect-triggered check landing
+    // during the same network blip that caused the reconnect, and failing
+    // -- see checkForUpdate's own comment).
+    function handleVisibilityChange() {
+      if (document.visibilityState === 'visible') checkForUpdate()
+    }
+    document.addEventListener('visibilitychange', handleVisibilityChange)
+    return () => document.removeEventListener('visibilitychange', handleVisibilityChange)
+  }, [])
+
   if (!needRefresh) return null
 
   return (

frontend/src/lib/swUpdate.ts

@@ -10,5 +10,15 @@ export function setSwRegistration(reg: ServiceWorkerRegistration): void {
 }
 
 export function checkForUpdate(): void {
-  void registration?.update()
+  // #58: this used to be a bare `void registration?.update()` -- if the
+  // fetch failed (most plausible right when it's triggered by a WS
+  // reconnect, i.e. the network just flapped from a backend restart), the
+  // rejection vanished with nothing to catch it and nothing logged. The
+  // only other trigger was an hourly interval, so a client that hit this
+  // at the wrong moment could sit stale for up to an hour with zero trace
+  // of why. This doesn't fix a bad network, but it stops the failure from
+  // being silent, and callers still don't need to handle anything.
+  registration?.update().catch((err: unknown) => {
+    console.error('Service worker update check failed', err)
+  })
 }