deps(frontend): jsdom 30 + testing-library/jest-dom 7 (lands #412/#409)

[WalksWithASwagger]

Summary

Lands both frontend test-environment major upgrades as one reviewable change, per the batching plan in #420 (frontend dev-test group):

• jsdom 29.1.1 → 30.0.0
• @testing-library/jest-dom 6.9.1 → 7.0.0

Supersedes Dependabot PRs #412 (jsdom) and #409 (jest-dom). A human should close #412 and #409 when this merges — this PR does not close them.

What changed

• frontend/package.json: the two devDependency version ranges.
• frontend/package-lock.json: regenerated to match.

No source or vitest config changes were required. The test setup already imports the framework-scoped matcher entry that jest-dom 7 requires:

// frontend/src/test/setup.ts
import "@testing-library/jest-dom/vitest";

and jsdom 30 works transparently under vitest 4 (environment: "jsdom"). jest-dom 7's type augmentation is still picked up via the existing tsconfig.json types: ["…","@testing-library/jest-dom"] entry, so tsc --noEmit stays clean.

Validation

Run in the worktree on the pushed tree (local Node 26; CI on Node 22.13 is the gate):

• npm --prefix frontend ci → clean, 0 vulnerabilities (strict lockfile in sync)
• npm --prefix frontend run test (vitest run) → 5 files, 10 tests passed
• npm --prefix frontend run typecheck (tsc --noEmit) → pass

Only jest-dom matchers in use are toBeInTheDocument and toHaveAttribute — both unaffected by the v7 major (which mainly drops legacy Node/entry-point support).

Scope / non-goals

• Only the two frontend test-env devDeps. No runtime deps, no other bumps, no unrelated code.
• Review-ready only — not auto-merged.

Refs #420