fix: geohash online count accuracy and channel switch latency (#672)

- Keep max lastSeen per user so relay's newest-first delivery doesn't
  overwrite a fresh timestamp with a stale one
- Serialize event processing on Main dispatcher to eliminate
  ConcurrentModificationException on geohashParticipants maps
- Immediately refresh people list on channel switch so sampling data
  is reflected before the first relay response arrives

app/src/main/java/com/bitchat/android/nostr/GeohashMessageHandler.kt

@@ -6,9 +6,7 @@ import com.bitchat.android.model.BitchatMessage
 import com.bitchat.android.ui.ChatState
 import com.bitchat.android.ui.MessageManager
 import kotlinx.coroutines.CoroutineScope
-import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.launch
-import kotlinx.coroutines.withContext
 import java.util.Date
 
 /**
@@ -44,7 +42,7 @@ class GeohashMessageHandler(
     }
 
     fun onEvent(event: NostrEvent, subscribedGeohash: String) {
-        scope.launch(Dispatchers.Default) {
+        scope.launch {
             try {
                 if (event.kind != NostrKind.EPHEMERAL_EVENT && event.kind != NostrKind.GEOHASH_PRESENCE) return@launch
                 val tagGeo = event.tags.firstOrNull { it.size >= 2 && it[0] == "g" }?.getOrNull(1)
@@ -65,9 +63,6 @@ class GeohashMessageHandler(
                 // Blocked users check (use injected DataManager which has loaded state)
                 if (dataManager.isGeohashUserBlocked(pubkey)) return@launch
 
-                // Update repository (participants, nickname, teleport)
-                // Update repository on a background-safe path; repository will post updates to LiveData
-                
                 // Update participant count (last seen) on BOTH Presence (20001) and Chat (20000) events
                 if (event.kind == NostrKind.GEOHASH_PRESENCE || event.kind == NostrKind.EPHEMERAL_EVENT) {
                     repo.updateParticipant(subscribedGeohash, pubkey, Date(event.createdAt * 1000L))
@@ -107,7 +102,7 @@ class GeohashMessageHandler(
                         if (hasNonce) NostrProofOfWork.calculateDifficulty(event.id).takeIf { it > 0 } else null
                     } catch (_: Exception) { null }
                 )
-                withContext(Dispatchers.Main) { messageManager.addChannelMessage("geo:$subscribedGeohash", msg) }
+                messageManager.addChannelMessage("geo:$subscribedGeohash", msg)
             } catch (e: Exception) {
                 Log.e(TAG, "onEvent error: ${e.message}")
             }

app/src/main/java/com/bitchat/android/nostr/GeohashRepository.kt

@@ -103,7 +103,16 @@ class GeohashRepository(
 
     fun updateParticipant(geohash: String, participantId: String, lastSeen: Date) {
         val participants = geohashParticipants.getOrPut(geohash) { mutableMapOf() }
-        participants[participantId] = lastSeen
+        // Cap to now: prevents future-timestamped events (clock skew / malicious created_at)
+        // from pinning lastSeen and blocking subsequent normal heartbeats.
+        // Also keeps max: relays send events newest-first, so subsequent older events for
+        // the same user must not overwrite a fresher lastSeen.
+        val now = Date()
+        val effective = if (lastSeen.after(now)) now else lastSeen
+        val existing = participants[participantId]
+        if (existing == null || effective.after(existing)) {
+            participants[participantId] = effective
+        }
         if (currentGeohash == geohash) refreshGeohashPeople()
         updateReactiveParticipantCounts()
     }

app/src/main/java/com/bitchat/android/ui/GeohashViewModel.kt

@@ -367,6 +367,7 @@ class GeohashViewModel(
                 Log.d(TAG, "📍 Switching to geohash channel: ${channel.channel.geohash}")
                 activeChannelGeohash = channel.channel.geohash
                 repo.setCurrentGeohash(channel.channel.geohash)
+                repo.refreshGeohashPeople()
                 notificationManager.setCurrentGeohash(channel.channel.geohash)
                 notificationManager.clearNotificationsForGeohash(channel.channel.geohash)
                 try { messageManager.clearChannelUnreadCount("geo:${channel.channel.geohash}") } catch (_: Exception) { }