Improve BLE connection stability and message reliability (#318)

* Remove sequence numbers from protocol

- Remove sequenceNumber field from BitchatPacket struct
- Update BinaryProtocol to not encode/decode sequence numbers (header size reduced from 17 to 13 bytes)
- Replace sequence-based duplicate detection with content-based using packet ID hash
- Update packet ID generation to use SHA256(senderID + timestamp + type + payload prefix)
- Remove all sequence tracking variables and methods
- Simplify duplicate detection to rely on timestamp and content hashing

* Fix connection stability issues

- Increase peer availability check interval from 5 to 15 seconds
- Fix availability logic to not mark connected peers as unavailable
- Add BLE connection keepalive timer (20s) to prevent iOS timeouts
- Fix missing delivery ACKs by passing peripheral context through Noise decryption
- Reduce identity announce frequency from 2 to 10 seconds minimum
- Remove unnecessary identity announces on connection
- Debounce identity cache keychain saves (2 second delay)
- Add message retry notification handler in ChatViewModel
- Fix version negotiation redundancy by checking existing negotiations
- Keep Noise sessions for already-connected peers

* Fix build errors in message retry handler

- Fix reference to 'displayedMessages' - should be 'messages'
- Fix sendMessage call signature to use individual parameters instead of message object
- Both iOS and macOS builds now succeed

* Fix remaining connection stability issues

- Fix duplicate identity announces with content-based deduplication
- Simplify peripheral mapping with cleaner temp ID to peer ID transitions
- Improve graceful leave detection across peer ID rotations
- Track previousPeerID from announcements to maintain state
- Add time-based cleanup for gracefully left peers

* Fix connection stability issues

- Add special duplicate detection for identity announces
- Simplify peripheral mapping with dedicated structure
- Improve graceful leave detection with peer ID rotation handling
- Track graceful leave timestamps for cleanup
- Transfer states properly during peer ID rotation

* Improve BLE connection stability and message reliability

- Increase peer availability timeout from 5s to 15s to prevent flapping
- Add BLE keepalive timer with 30s interval to maintain connections
- Fix missing delivery ACKs by passing peripheral context through decryption
- Reduce identity announce frequency from 2s to 10s minimum interval
- Add keychain save debouncing with 2s delay to prevent excessive writes
- Implement message retry system for failed deliveries to favorites
- Fix version negotiation redundancy by checking existing state
- Add special duplicate detection for identity announcements
- Implement graceful leave detection with peer ID rotation support
- Simplify peripheral mapping to reduce complexity
- Fix switch statement structure issues causing build errors

These changes significantly improve connection stability, eliminate peer availability flapping, and ensure reliable message delivery.

---------

Co-authored-by: jack <jackjackbits@users.noreply.github.com>

bitchat/Identity/SecureIdentityStateManager.swift

@@ -27,6 +27,11 @@ class SecureIdentityStateManager {
     // Thread safety
     private let queue = DispatchQueue(label: "bitchat.identity.state", attributes: .concurrent)
     
+    // Debouncing for keychain saves
+    private var saveTimer: Timer?
+    private let saveDebounceInterval: TimeInterval = 2.0  // Save at most once every 2 seconds
+    private var pendingSave = false
+    
     // Encryption key
     private let encryptionKey: SymmetricKey
     
@@ -72,16 +77,48 @@ class SecureIdentityStateManager {
         }
     }
     
+    deinit {
+        // Force save any pending changes
+        forceSave()
+    }
+    
     func saveIdentityCache() {
+        // Mark that we need to save
+        pendingSave = true
+        
+        // Cancel any existing timer
+        saveTimer?.invalidate()
+        
+        // Schedule a new save after the debounce interval
+        saveTimer = Timer.scheduledTimer(withTimeInterval: saveDebounceInterval, repeats: false) { [weak self] _ in
+            self?.performSave()
+        }
+    }
+    
+    private func performSave() {
+        guard pendingSave else { return }
+        pendingSave = false
+        
         do {
             let data = try JSONEncoder().encode(cache)
             let sealedBox = try AES.GCM.seal(data, using: encryptionKey)
-            _ = keychain.saveIdentityKey(sealedBox.combined!, forKey: cacheKey)
+            let saved = keychain.saveIdentityKey(sealedBox.combined!, forKey: cacheKey)
+            if saved {
+                SecureLogger.log("Identity cache saved to keychain", category: SecureLogger.security, level: .debug)
+            }
         } catch {
             SecureLogger.logError(error, context: "Failed to save identity cache", category: SecureLogger.security)
         }
     }
     
+    // Force immediate save (for app termination)
+    func forceSave() {
+        saveTimer?.invalidate()
+        if pendingSave {
+            performSave()
+        }
+    }
+    
     // MARK: - Identity Resolution
     
     func resolveIdentity(peerID: String, claimedNickname: String) -> IdentityHint {