Bump all safe dependencies

[kwent]

Summary

• Bumps all minor/patch dependencies: @babel/preset-env, mini-css-extract-plugin, postcss, terser-webpack-plugin, webpack, core-js
• Bumps major dev dependencies: babel-jest 30, babel-loader 10, copy-webpack-plugin 14, css-loader 7, jest 30, jest-environment-jsdom 30, postcss-import 16, postcss-loader 8, postcss-preset-env 11, webpack-cli 7
• All tests pass, production build succeeds

Supersedes

Closes #8, closes #9, closes #10, closes #11, closes #13, closes #15, closes #18, closes #19, closes #20, closes #21, closes #22

Not included (need separate review)

• Bump styled-components from 5.3.11 to 6.3.12 #12 styled-components 5→6 (major breaking runtime change)
• Bump react-dom from 18.3.1 to 19.2.4 #14 react-dom 18→19 (failing CI)
• Bump react from 18.3.1 to 19.2.4 #16 react 18→19 (failing CI)
• Bump @testing-library/react from 12.1.5 to 16.3.2 #17 @testing-library/react 12→16 (major jump, needs React 18+ compat review)

Test plan

• yarn test — 11 tests pass
• yarn build — production build succeeds