feat: make sync overwrite policy configurable

[chinrw]

Summary

• Add exact (default) and prefer-newer sync overwrite policies.
• Preserve source-authoritative restore and rollback behavior by default.
• Let prefer-newer skip stale overwrites when the target has a newer modification time.
• Apply the policy to pull and push plans, and reject duplicate manifest paths.

prefer-newer requires reasonably synchronized device clocks. Mirror deletion behavior is unchanged.

Darkatse/TT-Sync#3

可以来帮忙看看这个功能么，同步的时候可以保留最新的chat file而不是直接覆盖。

[Darkatse]

已经在2.2.0版本中解决。