Upgrade Babel to 8 #6375

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Overview

Dependabot PR #6013 bumped @babel/core 7.29.6 → 8.0.9, @babel/preset-env 7.29.5 → 8.0.7, @babel/plugin-transform-runtime 7.29.0 → 8.0.6 and @babel/plugin-syntax-import-assertions 7.28.6 → 7.29.7. It was closed unmerged because every webpack build and Jest run failed. Babel 8 removes preset-env's useBuiltIns, which our shared config still sets, and several build and test packages only accept Babel 7.

Complexity: Medium
Target branch: unstable

Context

• babel.config.js re-exports kolibri-build/babel.config, which sets useBuiltIns. On build(deps-dev): Bump the babel group across 1 directory with 4 updates #6013 both "Build frontend assets" and "Frontend tests" fail with:

  The 'useBuiltIns' option has been removed
  

• kolibri-build 1.0.0 depends on its own @babel/core 7.29.7, preset-env 7.29.7, plugin-transform-runtime 7.29.7 and babel-loader 10.1.1. With build(deps-dev): Bump the babel group across 1 directory with 4 updates #6013 applied, two Babel cores load: webpack's babel-loader runs on 7.29.7 while the root preset-env 8.0.7 expects 8.x.
• Packages whose peer range rejects @babel/core 8, per pnpm-lock.yaml:
  • babel-jest 29.7.0 (^7.8.0); 30.x accepts ^8.0.0-0
  • @vue/vue2-jest 29.2.6 (@babel/core: 7.x, babel-jest: 29.x), used for .vue files in jest_config/jest.conf.js
  • babel-core 7.0.0-bridge.0 (^7.0.0-0)
• Other Babel 8 breaking changes to check: modules: "auto" now assumes ESM support, and @babel/runtime-corejs3 no longer has corejs exports.

The Change

• Polyfill injection should move from useBuiltIns to babel-plugin-polyfill-corejs3, the replacement Babel 8 names, either in kolibri-build or overridden in Studio's babel.config.js.
• These must move to Babel 8 together, since each one's Babel 7 copy or peer range breaks the others: @babel/core, @babel/preset-env, @babel/plugin-transform-runtime, kolibri-build's Babel dependencies, babel-jest, @vue/vue2-jest, and babel-core (bridge), the last removed if nothing still needs it.
• @babel/plugin-syntax-import-assertions 7.29.7 is a minor bump and can land on its own.

Acceptance Criteria

• pnpm install resolves a single @babel/core major, with no unmet Babel peer warnings
• pnpm build succeeds, and the production bundle still includes the core-js polyfills it ships today for the browserslist targets
• pnpm test passes, including .vue component specs
• CI "Build frontend assets" and "Frontend tests" pass

AI usage

rtibblesbot (Claude Code) reviewed #6013, read its CI logs and the lockfile peer ranges, and drafted this issue.

Filed from #6013's review.