fix(theme-classic): prevent Firefox clipping sidebar label descenders

[xusnitdinov]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Fixes Firefox-on-Windows clipping of glyph descenders (g, p, q, y) in documentation sidebar link and category labels.

Those labels use line-clamp: 2 with overflow: hidden and Infima's line-height: 1.25. Firefox paints the -webkit-box clip boundary differently from Chromium, so descenders get cut off. Chromium and Firefox-on-macOS are fine.

Test Plan

• Open docs sidebar in Firefox on Windows and confirm descenders on labels like "Getting started" / "Configuring" are no longer clipped
• Confirm two-line truncation for long labels still works
• Spot-check Chromium to confirm no visual regression

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Affected styles:

• packages/docusaurus-theme-classic/src/theme/DocSidebarItem/Link/styles.module.css
• packages/docusaurus-theme-classic/src/theme/DocSidebarItem/Category/styles.module.css

Related issues/PRs

Closes #12299

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