Fix remembering 1.0x playback speed

[LeonardNJU]

Summary

Fix the remembered playback-rate logic so returning a video to 1.0x is persisted across video/page changes.

Problem

The current ratechange handlers only write localStorage.mvPlayRate when the rate is not 1. If a user has a remembered non-default rate (for example 2.0x) and switches back to 1.0x, the stored value remains 2. The next video then starts at 2.0x again.

The existing Z shortcut also uses mvPlayRate as the last fast rate, so simply persisting 1 there would break the 1x <-> fast rate toggle.

Fix

• Persist every valid current playback rate, including 1.0x, in mvPlayRate.
• Keep the last non-1x rate separately in mvLastPlayRate.
• Make the Z shortcut restore mvLastPlayRate, with the old mvPlayRate value as a migration fallback for existing users.
• When toggling from a fast rate to 1x, preserve that fast rate before persisting 1x.

This keeps the current Z shortcut behavior while making the "Remember video playback speed" option reflect the actual current rate.

Verification

• node --check 视频站h5.user.js
• State-transition test covering legacy mvPlayRate=2, switching to 1x, loading the next video at 1x, and toggling back to the previous 2x rate.