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/Protocols/BitchatProtocol.swift

@@ -139,9 +139,8 @@ struct BitchatPacket: Codable {
     let payload: Data
     let signature: Data?
     var ttl: UInt8
-    let sequenceNumber: UInt32  // New field for duplicate detection
     
-    init(type: UInt8, senderID: Data, recipientID: Data?, timestamp: UInt64, payload: Data, signature: Data?, ttl: UInt8, sequenceNumber: UInt32 = 0) {
+    init(type: UInt8, senderID: Data, recipientID: Data?, timestamp: UInt64, payload: Data, signature: Data?, ttl: UInt8) {
         self.version = 1
         self.type = type
         self.senderID = senderID
@@ -150,11 +149,10 @@ struct BitchatPacket: Codable {
         self.payload = payload
         self.signature = signature
         self.ttl = ttl
-        self.sequenceNumber = sequenceNumber
     }
     
     // Convenience initializer for new binary format
-    init(type: UInt8, ttl: UInt8, senderID: String, payload: Data, sequenceNumber: UInt32 = 0) {
+    init(type: UInt8, ttl: UInt8, senderID: String, payload: Data) {
         self.version = 1
         self.type = type
         // Convert hex string peer ID to binary data (8 bytes)
@@ -173,7 +171,6 @@ struct BitchatPacket: Codable {
         self.payload = payload
         self.signature = nil
         self.ttl = ttl
-        self.sequenceNumber = sequenceNumber
     }
     
     var data: Data? {