Add Simplified Chinese localization

[TsukasaHwan]

What

Added Simplified Chinese (zh-CN) localization support across the UI.

• Registered zh-CN in the i18n configuration and language settings.
• Added the Simplified Chinese locale and aligned its keys with en.json.
• Localized the Insights trigger/panel and navigation menu labels, tooltips, and accessibility labels.
• Updated the i18n documentation and language API example.
• Kept new bg.json entries as English placeholders.
• Added test coverage for selecting Simplified Chinese.

Testing

• Targeted Jest suites: 8 passed, 32 tests passed.
• ESLint passed for all modified UI files.
• Prettier checks passed for the modified locale files and UI files.
• Verified that en.json, bg.json, and zh-CN.json contain the same keys in the same order.
• No screenshots or recordings were added because the change is focused on localization and text content.

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Note

Low Risk
UI copy and locale registration only; no changes to auth, data handling, or connection logic.

Overview
Adds Simplified Chinese (zh-CN) as a shipped locale: i18n extract config, SUPPORTED_LANGUAGES, runtime resources, and a full zh-CN.json translation bundle aligned with en.json.

Several sidebar and Insights surfaces that still used hardcoded English are wired to t() (navigation help/notifications/cloud/GitHub, Insights panel tabs and trigger tooltips/ARIA). Matching keys land in en.json; bg.json gets English placeholders for the new keys. Settings language tests and Bruno PATCH /settings docs are updated for zh-CN.

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[CLAassistant]

All committers have signed the CLA.

[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 2 potential issues.

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[valkirilov]

Thanks for your effort @TsukasaHwan, and for the contribution. However, we're still not ready to accept this PR, there are a few places where we need to complete in order to have full support for i18n of all the strings in the app. Once this is done, we can start adding support for various languages.

Reference #6433