Cache geohash identity in ChatViewModel to prevent crypto during rendering

Additional optimization for location channels (voice notes are mesh-only,
but this helps with text message rendering in geohash channels):

- Add cachedGeohashIdentity to avoid deriveIdentity calls during rendering
- Check cache before falling back to crypto derivation
- Reduces main thread crypto work in location channels

bitchat/ViewModels/ChatViewModel.swift

@@ -363,6 +363,7 @@ final class ChatViewModel: ObservableObject, BitchatDelegate {
     private var geoSubscriptionID: String? = nil
     private var geoDmSubscriptionID: String? = nil
     private var currentGeohash: String? = nil
+    private var cachedGeohashIdentity: (geohash: String, identity: NostrIdentity)? = nil // Cache current geohash identity
     private var geoNicknames: [String: String] = [:] // pubkeyHex(lowercased) -> nickname
     // Show Tor status once per app launch
     private var torStatusAnnounced = false
@@ -3763,7 +3764,19 @@ final class ChatViewModel: ObservableObject, BitchatDelegate {
             if let spid = message.senderPeerID?.id {
                 // In geohash channels, compare against our per-geohash nostr short ID
                 if case .location(let ch) = activeChannel, spid.hasPrefix("nostr:") {
-                    if let myGeo = try? idBridge.deriveIdentity(forGeohash: ch.geohash) {
+                    // Use cached identity to avoid crypto during rendering
+                    let myGeo: NostrIdentity? = {
+                        if let cached = cachedGeohashIdentity, cached.geohash == ch.geohash {
+                            return cached.identity
+                        }
+                        // Fallback: derive and cache (should rarely happen)
+                        if let identity = try? NostrIdentityBridge.deriveIdentity(forGeohash: ch.geohash) {
+                            cachedGeohashIdentity = (ch.geohash, identity)
+                            return identity
+                        }
+                        return nil
+                    }()
+                    if let myGeo = myGeo {
                         return spid == "nostr:\(myGeo.publicKeyHex.prefix(TransportConfig.nostrShortKeyDisplayLength))"
                     }
                 }