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/_PreviewHelpers/PreviewKeychainManager.swift

@@ -10,32 +10,51 @@ import Foundation
 
 final class PreviewKeychainManager: KeychainManagerProtocol {
     private var storage: [String: Data] = [:]
+    private var serviceStorage: [String: [String: Data]] = [:]
     init() {}
-    
+
     func saveIdentityKey(_ keyData: Data, forKey key: String) -> Bool {
         storage[key] = keyData
         return true
     }
-    
+
     func getIdentityKey(forKey key: String) -> Data? {
         storage[key]
     }
-    
+
     func deleteIdentityKey(forKey key: String) -> Bool {
         storage.removeValue(forKey: key)
         return true
     }
-    
+
     func deleteAllKeychainData() -> Bool {
         storage.removeAll()
+        serviceStorage.removeAll()
         return true
     }
-    
+
     func secureClear(_ data: inout Data) {}
-    
+
     func secureClear(_ string: inout String) {}
-    
+
     func verifyIdentityKeyExists() -> Bool {
         storage["identity_noiseStaticKey"] != nil
     }
+
+    // MARK: - Generic Data Storage (consolidated from KeychainHelper)
+
+    func save(key: String, data: Data, service: String, accessible: CFString?) {
+        if serviceStorage[service] == nil {
+            serviceStorage[service] = [:]
+        }
+        serviceStorage[service]?[key] = data
+    }
+
+    func load(key: String, service: String) -> Data? {
+        serviceStorage[service]?[key]
+    }
+
+    func delete(key: String, service: String) {
+        serviceStorage[service]?.removeValue(forKey: key)
+    }
 }