chore(website): update dependencies and build on Node.js 26

[DutchyD]

Refresh the locked dependencies of the website to their latest versions within range, and override the packages that the latest Docusaurus, cssnano, sockjs and mermaid still pin to older majors. The website requires Node.js 26 and builds on it in CI.

Summary

Keeps the website's dependencies on their latest stable releases.

• npm update refreshes package-lock.json within the existing ranges.
• overrides in package.json moves these packages to their latest stable releases:

Package	Was	Now	Pulled in by
tinypool	1.1.1	2.2.0	@docusaurus/core
serialize-javascript	6.0.2	7.1.2	copy-webpack-plugin, css-minimizer-webpack-plugin
uuid	8.3.2	14.0.2	sockjs
postcss-selector-parser	6.1.4	7.1.6	cssnano plugins
katex	0.16.47	0.19.0	mermaid

• engines.node requires Node.js 26, and the website workflow builds on Node.js 26.

The direct dependencies were already at their latest stable versions and are unchanged.

Tests

• npm run typecheck passes.
• The website workflow runs npm ci, npm run typecheck and npm run build on Node.js 26.

Checklist

• The website describes the new behavior, following website/STYLE.md.
• Changed editor commands or shortcuts: npm run shortcuts was run in website/.
• Changed UI: the screenshots that show it were rendered again.
• Needs the slow tests: the run-slow label is on.