Swap jsdom for happy-dom test env (ui-extensions)

[amcaplan]

Swap jsdom → happy-dom test environment in ui-extensions packages

Why: Part of an initiative to cut low-value dependency churn (jsdom: ~2 Dependabot bumps / 24 months). happy-dom is a lighter, faster vitest browser environment with no feature gap for these packages' test suites.

What changed (devDep-only — 4 files):

• packages/ui-extensions-dev-console/vite.config.mts — vitest environment: 'jsdom' → 'happy-dom'
• packages/ui-extensions-server-kit/vite.config.mts — vitest environment: 'jsdom' → 'happy-dom'
• both package.json — removed "jsdom": "^25.0.0", added "happy-dom": "^20.10.1" to devDependencies
• patch changeset for @shopify/ui-extensions-server-kit (dev-console is "private": true)

Validation: All tests pass under happy-dom with no test-code changes — the swap is transparent.

Package	Tests	Lint
ui-extensions-dev-console	6 files / 28 tests ✅	✅
ui-extensions-server-kit	7 files / 61 tests ✅	✅

(server-kit type-check ✅. dev-console has pre-existing ReactNode type errors in .tsx files untouched by this change — a test-environment/devDep swap cannot affect those.)

Surface impact: devDependencies only — the published artifact of @shopify/ui-extensions-server-kit is unaffected.

🤖 AI-generated draft — needs human review before merge.