build(deps): upgrade Docusaurus 3.10, React 19, eslint 9 flat config

[darioAnongba]

Summary

Dependency alignment + upgrade for the docs site (mirrors the dashboard's modernization). Latest versions where the ecosystem allows.

Upgrades

• Docusaurus (@docusaurus/*) 3.5.2 → 3.10.1
• React 18 → 19 (Docusaurus 3.10 peer-supports it)
• TypeScript 5.5 → 5.9 — held below 6 on purpose (typescript-eslint 8 doesn't support TS 6 yet)
• @easyops-cn/docusaurus-search-local 0.44 → 0.55, @mdx-js/react → 3.1, prism-react-renderer → 2.4, clsx → 2.1, prettier → 3.8
• Tooling: Yarn 4.4 → 4.17

ESLint: 8 + airbnb → 9 flat config

eslint-config-airbnb / airbnb-typescript don't support eslint 9 and are effectively unmaintained. Replaced with a lean eslint 9 flat config (eslint.config.mjs) on typescript-eslint 8 + react / react-hooks / unused-imports + prettier-disable — consistent with the dashboard. Removed .eslintrc.js and the obsolete .eslintignore (eslint 9 uses the ignores key). The auto-removed React imports are the expected result of the JSX automatic runtime.

Notes

• Kept .yarnrc.yml minimal (enableScripts is required for core-js build scripts). Stripped the lines the Yarn migration auto-added — npmMinimalAgeGate: 0 and approvedGitRepositories: "**" — to avoid silently weakening supply-chain settings (the repo has no git deps).
• The remaining react@^18 peer warning comes from search-local's transitive Docusaurus plugin; the build runs the local search index fine.

Verification

✅ yarn install --immutable · yarn build · yarn typecheck · yarn lint · yarn fm:check — all green.

Part of the v0.2.0 docs alignment (#7). Content alignment with the current SwissKnife API/features follows in a separate PR.