Fix localized preview toolbar actions

[wadjj]

What changed

• resolve the Preview Mode Change Mode action name using the same locale mapping as MarkEdit-preview (Change Mode, 切换模式, or 切換模式)
• migrate an existing localized toolbar item to the active locale when setup is applied
• remove duplicate localized toggle items and make Revert recognize every supported action name
• document locale-aware behavior in the English and Korean READMEs
• add bundle-level regression tests for locale migration, deduplication, and Revert

Why

MarkEdit custom toolbar actions are resolved by their visible main-menu title. Direct Preview always wrote the English Change Mode title, while MarkEdit-preview registers localized titles for Simplified and Traditional Chinese. On a Chinese system, the eye toolbar button therefore could not find its action and did nothing.

User impact

The one-click preview toolbar button now works in English, Simplified Chinese, and Traditional Chinese. Existing configurations are migrated on the next Apply One-Click Toggle Setup… without creating duplicate buttons.

Validation

• ./node_modules/.bin/tsc --noEmit
• npm test (Vite production build plus 6 passing Node tests)