Implement offline-first submission sync engine with conflict resolution

Build a sync engine that queues form submissions locally when offline and syncs them in order when connectivity is restored, with conflict detection.

class SubmissionQueue {
  enqueue(submission: Submission): Promise<void>
  sync(): Promise<SyncResult>
  getStatus(): QueueStatus
}

// Submission: { instanceId: string, formId: string, data: Record<string, unknown>, submittedAt: string }
// SyncResult: { synced: number, failed: number, conflicts: ConflictRecord[], errors: string[] }
// QueueStatus: { pending: number, failed: number, lastSyncAt: string | null }
// ConflictRecord: { instanceId: string, resolution: 'last-write-wins', mergeLog: string }

Storage: persist the queue to IndexedDB (use idb library). Queue must survive page reloads.

Sync strategy:

1. Process queue FIFO order
2. POST each submission to /api/submissions
3. On 409 Conflict (same instanceId already exists): resolve via last-write-wins (the local version wins), re-POST with X-Conflict-Resolution: overwrite, log the conflict in ConflictRecord
4. On 5xx or network error: retry up to 3 times with exponential backoff (1s, 2s, 4s), then mark as failed
5. On success: remove from queue, update lastSyncAt

Event emitter: emit queued, syncing, synced, conflict, failed events so UI can show sync status.

Write unit tests for: queue persistence across simulated page reloads, conflict detection and resolution, partial sync recovery (fails on item 3 of 5, resumes from item 3).