ios: add chat fork actions

ios/Packages/Sybil/Sources/Sybil/SybilViewModel.swift

@@ -170,6 +170,7 @@ final class SybilViewModel {
     var isLoadingCollections = false
     var isLoadingSelection = false
     var isCreatingSearchChat = false
+    var isForkingChat = false
     var chatBottomPinRequestID = 0
     var errorMessage: String?
 
@@ -958,6 +959,36 @@ final class SybilViewModel {
         }
     }
 
+    @discardableResult
+    func forkChat(chatID: String, messageID: String? = nil) async -> ChatSummary? {
+        guard isAuthenticated, !isForkingChat else {
+            return nil
+        }
+
+        guard messageID != nil || !isChatRowRunning(chatID) else {
+            errorMessage = "Wait for the response to finish before forking this chat."
+            return nil
+        }
+
+        let forkDescription = messageID.map { "chat \(chatID) from response \($0)" } ?? "chat \(chatID)"
+        SybilLog.info(SybilLog.ui, "Forking \(forkDescription)")
+        errorMessage = nil
+        isForkingChat = true
+        defer {
+            isForkingChat = false
+        }
+
+        do {
+            let forkedChat = try await client().forkChat(chatID: chatID, messageID: messageID)
+            applyChatSummary(forkedChat, moveToFront: true)
+            return forkedChat
+        } catch {
+            errorMessage = normalizeAPIError(error)
+            SybilLog.error(SybilLog.ui, "Fork failed", error: error)
+            return nil
+        }
+    }
+
     func renameChat(chatID: String, title: String) async {
         guard isAuthenticated else {
             return