feat: automatic offline sync - rebroadcast local events when back online

- Create offlineSyncService to sync local-only events to remote relays
- Create useOfflineSync hook to detect online/offline transitions
- When user comes back online (remote relays connect), automatically:
  - Query local relays for user's events from last 24 hours
  - Rebroadcast highlights and bookmarks to remote relays
- Integrate sync into Bookmarks component
- Enables seamless offline workflow:
  - User can work offline with local relays
  - Events are automatically synced when connection restored
  - No manual intervention required

src/hooks/useOfflineSync.ts

@@ -0,0 +1,57 @@
+import { useEffect, useRef } from 'react'
+import { RelayPool } from 'applesauce-relay'
+import { IAccount } from 'applesauce-core/helpers'
+import { syncLocalEventsToRemote } from '../services/offlineSyncService'
+import { isLocalRelay } from '../utils/helpers'
+import { RelayStatus } from '../services/relayStatusService'
+
+interface UseOfflineSyncParams {
+  relayPool: RelayPool | null
+  account: IAccount | null
+  relayStatuses: RelayStatus[]
+  enabled?: boolean
+}
+
+export function useOfflineSync({
+  relayPool,
+  account,
+  relayStatuses,
+  enabled = true
+}: UseOfflineSyncParams) {
+  const previousStateRef = useRef<{
+    hasRemoteRelays: boolean
+    initialized: boolean
+  }>({
+    hasRemoteRelays: false,
+    initialized: false
+  })
+
+  useEffect(() => {
+    if (!enabled || !relayPool || !account) return
+
+    const connectedRelays = relayStatuses.filter(r => r.isInPool)
+    const hasRemoteRelays = connectedRelays.some(r => !isLocalRelay(r.url))
+    const hasLocalRelays = connectedRelays.some(r => isLocalRelay(r.url))
+
+    // Skip the first check to avoid syncing on initial load
+    if (!previousStateRef.current.initialized) {
+      previousStateRef.current = {
+        hasRemoteRelays,
+        initialized: true
+      }
+      return
+    }
+
+    // Detect transition: from local-only to having remote relays
+    const wasLocalOnly = !previousStateRef.current.hasRemoteRelays && hasLocalRelays
+    const isNowOnline = hasRemoteRelays
+
+    if (wasLocalOnly && isNowOnline) {
+      console.log('✈️ Detected transition: Flight Mode → Online')
+      syncLocalEventsToRemote(relayPool, account, true)
+    }
+
+    previousStateRef.current.hasRemoteRelays = hasRemoteRelays
+  }, [relayPool, account, relayStatuses, enabled])
+}
+