Add 5-second undo for accepted suggestions

[dushaobindoudou]

Background

When you accept a suggestion, the receipt only shows for 1.1s — too fast to undo. Gmail-style 5-second undo would dramatically improve trust.

Acceptance

• Receipt stays for 5 seconds (configurable constant)
• Receipt has "撤销 / Undo" link reverting the accept
• Auto-dismiss after 5s if no undo

Full brief: https://github.com/dushaobindoudou/ovo/blob/main/docs/GOOD_FIRST_ISSUES.md (issue #8)

⏱ ~1-2 h · 🏷 philosophy:teachable