feat(api): add onProgress callback to copy() and scan() APIs

[gregpriday]

Summary

Implements the onProgress callback option for copy() and scan() APIs, enabling consumers to show real-time progress bars without needing to construct the pipeline manually.

Closes #64

Changes Made

• src/utils/ProgressTracker.js (new) — normalizes pipeline events (stage:start, stage:complete, file:batch, stage:progress) into a simple { percent, message } format; throttled (default 100ms); monotonically increasing; exceptions in the callback are swallowed so progress tracking never breaks the operation
• src/api/scan.js — wires ProgressTracker into the pipeline when options.onProgress is provided; respects options.progressThrottleMs
• src/api/copy.js — adds emitProgress wrapper with monotonic guard and try/catch; scan phase covers 0–80%, format phase 80–100%; always emits 0% on start and 100% on complete
• src/index.js — exports ProgressTracker for advanced consumers
• types/index.d.ts — adds ProgressTracker class, ProgressTrackerOptions interface; totalStages is optional; onProgress/progressThrottleMs already present in ScanOptions/CopyOptions
• tests/unit/utils/ProgressTracker.test.js (new) — 24 unit tests covering lifecycle, monotonic enforcement, throttling, edge cases (null lastFile, missing message, throwing callbacks)
• tests/integration/onProgress.test.js (new) — 12 integration tests for scan() and copy() including dry run, throttle, edge cases

Usage

import { copy } from 'copytree';

const result = await copy('./large-repo', {
  onProgress: ({ percent, message }) => {
    mainWindow.webContents.send('copy-progress', { percent, message });
  }
});