refactor: consolidate KeychainHelper into KeychainManager (#797)

Merge KeychainHelper functionality into KeychainManager to provide
a single, unified API for all keychain operations.

- Remove KeychainHelper.swift and KeychainHelperProtocol
- Add generic save/load/delete methods to KeychainManagerProtocol
- Update NostrIdentityBridge to use KeychainManagerProtocol
- Update FavoritesPersistenceService to use KeychainManagerProtocol
- Update PreviewKeychainManager with new methods
- Update MockKeychain and add MockKeychainHelper typealias for backwards compatibility

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

bitchat/Nostr/KeychainHelper.swift

@@ -1,50 +0,0 @@
-import Foundation
-
-protocol KeychainHelperProtocol {
-    func save(key: String, data: Data, service: String, accessible: CFString?)
-    func load(key: String, service: String) -> Data?
-    func delete(key: String, service: String)
-}
-
-/// Keychain helper for secure storage
-struct KeychainHelper: KeychainHelperProtocol {
-    func save(key: String, data: Data, service: String, accessible: CFString? = nil) {
-        var query: [String: Any] = [
-            kSecClass as String: kSecClassGenericPassword,
-            kSecAttrService as String: service,
-            kSecAttrAccount as String: key,
-            kSecValueData as String: data
-        ]
-        if let accessible = accessible {
-            query[kSecAttrAccessible as String] = accessible
-        }
-        
-        SecItemDelete(query as CFDictionary)
-        SecItemAdd(query as CFDictionary, nil)
-    }
-    
-    func load(key: String, service: String) -> Data? {
-        let query: [String: Any] = [
-            kSecClass as String: kSecClassGenericPassword,
-            kSecAttrService as String: service,
-            kSecAttrAccount as String: key,
-            kSecReturnData as String: true
-        ]
-        
-        var result: AnyObject?
-        let status = SecItemCopyMatching(query as CFDictionary, &result)
-        
-        guard status == errSecSuccess else { return nil }
-        return result as? Data
-    }
-    
-    func delete(key: String, service: String) {
-        let query: [String: Any] = [
-            kSecClass as String: kSecClassGenericPassword,
-            kSecAttrService as String: service,
-            kSecAttrAccount as String: key
-        ]
-        
-        SecItemDelete(query as CFDictionary)
-    }
-}

bitchat/Nostr/NostrIdentityBridge.swift

@@ -12,9 +12,9 @@ final class NostrIdentityBridge {
     private var derivedIdentityCache: [String: NostrIdentity] = [:]
     private let cacheLock = NSLock()
 
-    private let keychain: KeychainHelperProtocol
+    private let keychain: KeychainManagerProtocol
 
-    init(keychain: KeychainHelperProtocol = KeychainHelper()) {
+    init(keychain: KeychainManagerProtocol = KeychainManager()) {
         self.keychain = keychain
     }