feat: implementation of offline-first sync engine poc (#13)

poc-sync-engine/.gitignore

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+node_modules/
+dist/
+coverage/

poc-sync-engine/README.md

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+# Offline-First Sync Engine (PoC)
+
+This directory contains the Proof of Concept (PoC) for Issue #1: **Offline-First Sync Engine: Queue-Based Write Architecture**.
+
+As per the contribution guidelines, this PoC is built as a **standalone TypeScript module**. It does not pollute the main repository scaffolding, allowing it to be unit-tested and reviewed in isolation before being integrated into the React Native app.
+
+## Architecture & Sequence
+
+This module handles the core complexities of the offline-first requirement:
+1. Enqueuing local writes.
+2. Waiting for network restoration.
+3. Automatically draining the queue with exponential backoff on failure.
+
+### Sequence Diagram
+
+```mermaid
+sequenceDiagram
+    participant UI as React Native UI
+    participant Queue as SyncQueue
+    participant NetInfo as Network Monitor
+    participant Manager as SyncManager
+    participant Frappe as Frappe REST API
+
+    UI->>Queue: enqueue(mutation)
+    Note over Queue: Saved to persistent storage
+
+    NetInfo->>Manager: updateNetworkStatus(true)
+    Manager->>Queue: peek()
+    Queue-->>Manager: returns job
+
+    Manager->>Frappe: POST /api/resource/...
+
+    alt Success
+        Frappe-->>Manager: 200 OK
+        Manager->>Queue: dequeue() (removes job)
+    else Failure (Network Drop)
+        Frappe--xManager: Timeout / Error
+        Manager->>Queue: requeue() (increments retry)
+        Note over Manager: Applies Exponential Backoff
+    end
+```
+
+## Running the PoC Tests
+
+Since this is a standalone logic module, we verify its behavior using Jest.
+
+1. Install dependencies:
+   ```bash
+   cd poc-sync-engine
+   npm install
+   ```
+2. Run the unit & simulation tests:
+   ```bash
+   npm test
+   ```
+
+## Files
+- `src/SyncQueue.ts` - Manages the FIFO queue logic.
+- `src/SyncManager.ts` - Handles the network state monitoring, draining, and exponential backoff retry logic.
+- `tests/SyncManager.test.ts` - Jest tests simulating offline/online transitions.

poc-sync-engine/jest.config.js

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+module.exports = {
+  preset: 'ts-jest',
+  testEnvironment: 'node',
+};