feat(nav): render Preview badges consistently

[qiancai]

Purpose

Render PREVIEW labels consistently across documentation navigation, including TOC image tags and the TiDB for AI left navigation title, using the existing neutral, localized Preview badge style.

Scope

• Detect PREVIEW TOC tags case-insensitively and render a fixed neutral badge style.
• Reuse the existing PreviewBadge for the TiDB for AI directory title.
• Remove the now-unused BetaTagBadge component and Beta locale entries.
• Preserve the generic BETA and custom tag color behavior for TOC compatibility.
• Avoid generating a ?undefined tag query for TOC image URLs without query parameters.
• Add TOC parser coverage for image tags with and without query parameters or alt text.

Validation

• Prettier checks for updated components, parser, tests, and locale files.
• JSON parsing for English, Chinese, and Japanese locale files.
• Runtime assertion that TiDB for AI uses the shared PreviewBadge and preview translation key.
• Jest coverage for the TOC tag parser and existing TOC namespace behavior.

Notes

PREVIEW TOC tags intentionally ignore a source color query parameter so their badge appearance remains consistent.

[vercel]

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[gemini-code-assist]

Code Review

This pull request fixes an issue where TOC images without query parameters would emit a bogus ?undefined query, adds corresponding unit tests, and introduces translation support for the 'PREVIEW' badge in the left navigation tree. The review feedback focuses on improving type safety by avoiding unsafe non-null assertions on image.alt, and optimizing the LeftNavTree component by leveraging MUI's built-in theme resolution for custom palette colors, which simplifies the generateItemLabel function signature and prevents potential runtime errors.

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