feat(connectivity): implement BrowserNetworkService using Web Connectivity APIs

[rohansaini-02]

Problem

Currently, poc-connectivity only implements MockNetworkService. While useful for local test suites, the application needs a production-ready browser implementation that monitors real browser network status.

Proposed Solution

Create a new BrowserNetworkService implementing INetworkService using web APIs:

• Monitor status using navigator.onLine.

  [!NOTE]
  The initial implementation can rely on navigator.onLine, while keeping the abstraction flexible for future enhancements such as active reachability checks.

• Listen to live state changes using the window online and offline event listeners.
• Notify registered subscribers whenever browser connectivity state changes.
• The implementation should ensure browser event listeners are cleaned up appropriately when the service is disposed or no longer needed.

export class BrowserNetworkService implements INetworkService {
  private listeners: Set<NetworkChangeListener> = new Set();

  constructor() {
    if (typeof window !== 'undefined') {
      window.addEventListener('online', () => this.handleStatusChange());
      window.addEventListener('offline', () => this.handleStatusChange());
    }
  }
  ...
}

Acceptance Criteria

• BrowserNetworkService is exposed by the connectivity module.
• The service reacts to browser online/offline events and exposes connectivity state through the existing subscription mechanism.