v0.2: Network heartbeat for unreliable navigator.onLine

[mayrang]

Problem

`navigator.onLine === true` lies in two common scenarios:

• Captive portals (coffee shop wifi, airport, hotel) — connected to wifi, no internet
• Airplane mode partial disable — wifi shows connected but data blocked

formdraft currently consults `navigator.onLine` for sync gating. When it lies, sync retries fire and fail, exhausting the retry budget without the user understanding why.

Proposed

A pluggable network detector option. Library default stays `navigator.onLine`; consumers can plug in a heartbeat-based detector.

API sketch

```tsx
import { createHeartbeatDetector } from 'formdraft';

const detector = createHeartbeatDetector({
url: '/api/health',
intervalMs: 30000,
timeoutMs: 5000,
});

useFormDraft({
// ...
onlineDetector: detector,
});
```

Or simpler — just a function:

```tsx
useFormDraft({
isOnline: async () => {
const r = await fetch('/health', { signal: AbortSignal.timeout(5000) }).catch(() => null);
return r?.ok ?? false;
},
});
```

Implementation notes

• `createHeartbeatDetector` returns an object that wraps internal interval + cached online state
• Sync queue consults the detector instead of `navigator.onLine` directly
• Default behavior unchanged when no detector provided
• Should respect `visibilitychange` (don't heartbeat in background tabs)

Acceptance

• New `createHeartbeatDetector` exported from main
• `useFormDraft` accepts `onlineDetector` option
• Sync queue uses detector when provided
• 3+ unit tests: heartbeat probes, detects captive portal failure, recovery on real-online
• README new section "Reliable online detection"