feat: implement network connectivity monitor interface

[rohansaini-02]

Context

Ref: #1 — Native GovTech Learning App for Public Education Systems

TAP Buddy is designed as an offline-first mobile application targeting low-connectivity environments. Core systems like the Sync Engine require real-time awareness of the device's network state to function correctly.

Problem

Currently, there is no standardized way to check network connectivity across the application.

1. Duplicated Logic — Individual modules might write their own network checks, violating the DRY principle.
2. Hard-to-Test Code — Directly querying network status makes it difficult to simulate offline scenarios in unit tests.
3. Tight Coupling — Relying directly on external libraries ties our core logic to specific implementations.

Proposed Solution

Create a dedicated INetworkService interface and a mock implementation in a new shared-utils/network directory.

Core Components

Component	Purpose
INetworkService	An abstract interface exposing methods like isOnline().
MockNetworkService	A mock implementation that allows developers to toggle online/offline state for testing.

Deliverables

• INetworkService interface is defined.
• MockNetworkService implementation is provided.
• Code resides in an independent shared-utils/network directory to prevent merge conflicts.

Why This Matters

This acts as a foundational utility. By using an interface, we can cleanly swap the mock implementation for the real @react-native-community/netinfo later without rewriting the Sync Engine.