docs(changelog): add missing v1.20.1 user-facing entries (#727)

Add four entries to v1.20.1 changelog that were present in the beta
commits but missing from the changelog file: error message translate
button, Tavily parse_link tool, Mermaid upgrade with Venn diagram
support, and input box font size adjustment.

zh_Hant changelog still tracks v1.15.4 and is left untouched.

https://claude.ai/code/session_01UDPCDpSFT7h9ZQEZTkfsTs

Co-authored-by: Claude <noreply@anthropic.com>
(cherry picked from commit 5a06f7a3ddd177c62a3c23f5d857e2a7bb993158)