add sse client for real-time backend events

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

clients/macos/MagnumOpus/Services/SSEClient.swift

@@ -0,0 +1,67 @@
+import Foundation
+
+struct ServerEvent {
+	let id: String?
+	let event: String?
+	let data: String
+}
+
+@Observable
+final class SSEClient {
+	private let url: URL
+	private var task: Task<Void, Never>?
+
+	var lastEvent: ServerEvent?
+
+	init(url: URL) {
+		self.url = url
+	}
+
+	func connect(onEvent: @escaping (ServerEvent) -> Void) {
+		task = Task {
+			do {
+				var request = URLRequest(url: url)
+				request.setValue("text/event-stream", forHTTPHeaderField: "Accept")
+
+				let (bytes, _) = try await URLSession.shared.bytes(for: request)
+
+				var currentEvent: String?
+				var currentData = ""
+				var currentId: String?
+
+				for try await line in bytes.lines {
+					if line.isEmpty {
+						if !currentData.isEmpty {
+							let event = ServerEvent(
+								id: currentId,
+								event: currentEvent,
+								data: currentData.trimmingCharacters(in: .newlines)
+							)
+							self.lastEvent = event
+							onEvent(event)
+							currentEvent = nil
+							currentData = ""
+							currentId = nil
+						}
+					} else if line.hasPrefix("data: ") {
+						currentData += String(line.dropFirst(6)) + "\n"
+					} else if line.hasPrefix("event: ") {
+						currentEvent = String(line.dropFirst(7))
+					} else if line.hasPrefix("id: ") {
+						currentId = String(line.dropFirst(4))
+					}
+				}
+			} catch {
+				if !Task.isCancelled {
+					try? await Task.sleep(for: .seconds(5))
+					connect(onEvent: onEvent)
+				}
+			}
+		}
+	}
+
+	func disconnect() {
+		task?.cancel()
+		task = nil
+	}
+}