Implement endpoint to get last finished sync mts for ui/ux

[ZIMkaRU]

This PR implements endpoint to get the last finished sync mts for the UI/UX

---

Basic changes:

• Add getLastFinishedSyncMts endpoint
• Add test coverage for getLastFinishedSyncMts method

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Request example:

{
  "auth": {
    "token": "user-token"
  },
  "method": "getLastFinishedSyncMts"
}

Response example:

{
  "jsonrpc": "2.0",
  "result": {
    "lastSyncMts": 1720504804336 // or `null` if never synced
  },
  "id": null
}

[ezewer]

lgtm!