Remove message retention and /save command

- Delete MessageRetentionService.swift
- Remove retentionEnabledChannels from ChatViewModel
- Remove /save command handling
- Remove retention UI elements from ContentView
- Remove channelRetention message type from protocol
- Update documentation and tests

bitchatTests/KeychainIntegrationTests.swift

@@ -97,12 +97,10 @@ class KeychainIntegrationTests: XCTestCase {
     func testNoPlaintextInUserDefaults() {
         // Create services to generate keys
         _ = NoiseEncryptionService()
-        _ = MessageRetentionService.shared
         
         // Check UserDefaults for any sensitive data
         let keysToCheck = [
             "bitchat.noiseIdentityKey",
-            "bitchat.messageRetentionKey",
             "bitchat.channelPasswords",
             "bitchat.identityKey",
             "bitchat.staticKey"

bitchatTests/NoiseIdentityPersistenceTests.swift

@@ -112,31 +112,7 @@ class NoiseIdentityPersistenceTests: XCTestCase {
     }
     
     // MARK: - Message Retention Key Tests
-    
-    func testMessageRetentionKeyPersistence() {
-        // Create first instance
-        _ = MessageRetentionService.shared
-        
-        // Get key from keychain
-        let keyData1 = KeychainManager.shared.getIdentityKey(forKey: "messageRetentionKey")
-        XCTAssertNotNil(keyData1, "Message retention key should be stored")
-        
-        // Simulate app restart by clearing the singleton
-        // (In real app, this would be a new process)
-        
-        // Get key again
-        let keyData2 = KeychainManager.shared.getIdentityKey(forKey: "messageRetentionKey")
-        XCTAssertEqual(keyData1, keyData2, "Message retention key should persist")
-    }
-    
-    func testMessageRetentionKeyNotInUserDefaults() {
-        // Ensure service is initialized
-        _ = MessageRetentionService.shared
-        
-        // Verify key is NOT in UserDefaults
-        let userDefaultsData = UserDefaults.standard.data(forKey: "bitchat.messageRetentionKey")
-        XCTAssertNil(userDefaultsData, "Message retention key should NOT be in UserDefaults")
-    }
+    // Message retention feature has been removed
     
     // MARK: - Keychain Service Name Tests