fix(desktop): localize appearance settings in zh-CN

[lihuiyang1024]

Description

Follow-up to #154 and #187.

The zh-CN locale currently translates the Appearance section heading, but the controls beneath it remain in English. This completes that high-visibility Settings surface while preserving inline English strings as the source-of-truth fallbacks.

Changes Made

• localize the appearance mode cards and their accessible group label
• localize theme names, conversation width, font size, and preferred-font controls
• localize installed-font groups, guidance, local-only note, and reset action
• extend the existing language-switch integration test to cover the Appearance surface

Verification

• npm run typecheck
• npm run lint
• npm test — 31 files, 195 tests passed
• npm run build

Checklist

• Changes tested locally
• Code reviewed
• Documentation updated (not required; UI copy only)
• Unit tests added

[Zongwei9888]

Merged today as e40c665 (via "Merge PR #202"). With this and #187 the Desktop shell is essentially fully localized for zh-CN; the computed theme keys and the extended language-switch test both read well. Thank you.