feat: implement network connectivity monitor interface and mock service (#26)

poc-connectivity/.gitignore

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+node_modules/
+dist/
+coverage/
+.env
+*.log

poc-connectivity/README.md

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+# POC: Connectivity Monitor Utility
+
+## Overview
+This module provides a decoupled, interface-driven approach to monitoring network connectivity in the TAP Buddy application. By abstracting the network state behind `INetworkService`, we ensure that the core application logic (such as the Sync Engine) remains agnostic of the specific library used for network detection (e.g., `@react-native-community/netinfo`).
+
+## Architecture
+
+### Interface-Driven Design
+The module defines a strict `INetworkService` interface:
+```typescript
+export interface INetworkService {
+  getNetworkState(): NetworkState;
+  isOnline(): boolean;
+  subscribe(listener: NetworkChangeListener): () => void;
+}
+```
+
+### Mock Implementation
+A `MockNetworkService` is provided to facilitate development and testing without requiring a physical device or real network transitions.
+
+```mermaid
+sequenceDiagram
+    participant App as Application Logic
+    participant Mock as MockNetworkService
+    participant Listener as NetworkListener
+
+    App->>Mock: subscribe(listener)
+    Mock-->>Listener: notify(initialState)
+
+    Note over Mock: simulateOffline() triggered
+
+    Mock-->>Listener: notify(isConnected: false)
+    App->>Mock: isOnline()
+    Mock-->>App: false
+```
+
+## Why This Matters
+- **Testability:** Allows simulating flaky connections and offline states in unit tests.
+- **Maintainability:** If we decide to switch network monitoring libraries in the future, we only need to implement a new version of `INetworkService` without touching the rest of the codebase.
+- **Decoupling:** Prevents the Sync Engine from being tightly coupled to React Native native modules during the PoC phase.
+
+## Usage
+```typescript
+const network = new MockNetworkService();
+
+network.subscribe((state) => {
+  console.log('Connection status changed:', state.isConnected);
+});
+
+network.simulateOffline(); // Triggers listener
+```

poc-connectivity/jest.config.js

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+module.exports = {
+  preset: 'ts-jest',
+  testEnvironment: 'node',
+  testMatch: ['**/tests/**/*.test.ts'],
+  collectCoverage: true,
+  coverageDirectory: 'coverage',
+  coverageThreshold: {
+    global: {
+      branches: 90,
+      functions: 90,
+      lines: 90,
+      statements: 90
+    }
+  }
+};