Add Traditional Chinese language family

[BearNetwork-BRNKC]

Here is the PR description in full English:

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Why

The existing Chinese locale (zh) only supported Simplified Chinese, with no option for Traditional Chinese users. This PR splits the single Chinese locale into two distinct options:

• Simplified Chinese (zh-CN) — retains all existing translations, previously labelled as 中文, now clearly labelled as 簡體中文 (Simplified Chinese)
• Traditional Chinese (zh-TW) — new locale with full Taiwan-localized translations (e.g. using 預設 instead of 默认, 網路 instead of 网络, 通訊埠 instead of 端口, 儲存 instead of 保存, etc.)

Each locale displays its own country flag (🇨🇳 for Simplified Chinese, 🇹🇼 for Traditional Chinese) in the locale picker.

Help documentation has been separated and fully translated for each variant. HelpModal is updated to use the full locale code (e.g. zh-TW) instead of the short code (e.g. zh), so Traditional Chinese users correctly see Traditional Chinese help docs rather than falling back to Simplified Chinese or English.

A prefix-based fallback is also added to getHelpFile() so that any locale using a short code (e.g. zh) continues to resolve gracefully to the most relevant available help document.

All changes are non-breaking. No existing locale or functionality is removed or altered beyond the zh locale rename/split.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• API changes
• Performance improvement
• Test addition or update

AI Usage

• [] AI was used to write this
• [] AI was used to review this

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Files changed summary:

File	Change
frontend/src/locale/src/lang-list.json	Split zh-CN label, add zh-TW entry
frontend/src/locale/src/zh.json → zh_cn.json	Renamed, content unchanged
frontend/src/locale/src/zh_tw.json	New — full Taiwan-localized Traditional Chinese translations
frontend/src/locale/IntlProvider.tsx	Register both locales, add zh-TW → TW flag mapping
frontend/src/locale/src/HelpDoc/zh/ → zh-cn/	Renamed directory
frontend/src/locale/src/HelpDoc/zh-tw/	New — Traditional Chinese help documents
frontend/src/locale/src/HelpDoc/index.ts	Import both variants, add prefix fallback in getHelpFile()
frontend/src/modals/HelpModal.tsx	Use full locale code for help doc lookup
frontend/src/locale/Utils.test.tsx	Add zh-TW flag code test case
frontend/check-locales.cjs	Register zh-CN and zh-TW in locale validator

[BearNetwork-BRNKC]

These 5 failing tests are unrelated to the changes in this PR, which only modifies frontend locale files (translations, language picker logic, and help documentation).

The failing tests cover backend certificate management, Powerdns DNS provisioning, and TCP stream proxying — none of which are touched by this PR. Could a maintainer please verify whether these tests are also failing on the main branch, and re-run or manually approve the CI if so? Thank you!