[v0lumehi]

Fix or feature

• Fix
• Feature

What and why

Release prep for the release candidate v1.2.1-rc1. No tag is set here.

• appVersion is "1.2.1" in AppUpdateStore.swift. That is the only place the 1.2.0 prep touched besides the notes.
• release-notes/v1.2.1.md is new, English first, Chinese second, no generated block (CI appends it).
• A row for v1.2.1 in CHANGELOG.md and CHANGELOG.zh-CN.md.

The notes cover #783, #722 to #728. #729 and #731 are open and not in them yet.

Checked

• Tests: pre-push hook passed (Qt front end, Mac launcher swift test, tools, engine sync). python3 tools/release_notes.py check release-notes/v1.2.1.md is ok. Workflow script tests pass.
• Test for a fix: none, notes and version only.
• Visual QA: not applicable.
• Run on a device or Mac: not applicable.

Screenshots

None, no visible change.

Scope

• No engine file (dm/, scripts/, docs/, ref/) is added to the repository
• No engine or game-profile change needs an upstream PR
• No visible launcher-view change needs screenshots
• No keys, game files or data extracted from a game in the change or the attachments