Fix SSE reconnection when app returns to foreground

[vanpelt]

Problem

When users navigated away from the app during an SSE connection to a codespace, iOS would suspend the URLSession connection. Upon returning to the app, the connection would remain stuck in a "connecting" state indefinitely, never completing or showing an error.

Solution

Added app lifecycle monitoring using SwiftUI's scenePhase environment value. When the app is backgrounded during an active SSE connection, the view now tracks this state. Upon returning to the foreground, it automatically disconnects the stale connection and initiates a fresh SSE connection to the same codespace.

Implementation

• Added @Environment(\.scenePhase) to monitor app state transitions
• Tracks connection state with wasConnectingBeforeBackground flag
• Stores pendingCodespaceName to preserve the target codespace across app backgrounding
• Implements handleScenePhaseChange() to detect background→active transitions and trigger reconnection
• Uses SwiftUI-native approach instead of NotificationCenter for better integration with view lifecycle