Add support for syncing existing/ongoing threads

[YastecHub]

Problem

This tool has two user flows that need different setup:

1. Fresh start users - Starting their first LeetCode thread (covered by start_thread.py)
2. Existing thread users - Already posting daily on X, need to sync this tool to their ongoing thread (NOT covered)

Currently, users with existing threads are confused because:

• start_thread.py would create a NEW thread (losing their existing one)
• No safe way to initialize without overwriting progress
• Manual progress.json creation is error-prone

Current Behavior

• Only start_thread.py exists for initialization
• It assumes users are starting fresh
• No detection/handling for existing threads

Expected Behavior

• Clear path for users with existing threads
• Safe initialization without creating duplicate threads
• Automatic thread ID extraction and day tracking

Proposed Solution

Create a separate sync_existing_thread.py script for users with ongoing threads:

• Prompts: "Do you have an existing thread?"
• If yes: asks for first tweet URL
• Auto-extracts thread ID
• Asks current day number
• Generates progress.json

This provides a distinct entry point so existing thread users don't accidentally use start_thread.py and lose their thread.

I'd be happy to contribute a PR for this feature!