chore(deps): update Vitest to v5, jsdom to v30 and Node.js to 26

[1natsu172]

Summary

• Update Vitest packages (vitest, @vitest/coverage-v8, @vitest/ui) from 4.1.11 to 5.0.2, as the groundwork for test: run the test suite on real browsers with Vitest browser mode #210. The lockfile now resolves vite@8.3.1, which satisfies Vitest 5's vite >= 6.4 requirement.
• Update jsdom from 29.1.1 to 30.1.1 and @types/jsdom from 28.0.3 to 30.0.0.
• Update Node.js from 24.20.0 to 26.10.0 in .tool-versions. CI already reads this file via node-version-file, so local and CI use the same version.

typescript and unbuild are out of scope (see #218). Vitest 5.0.3 is skipped for now because it is still inside the 7-day minimum release age; Renovate will pick it up later.

Closes #211

Breaking changes checked

• Vitest 5 (migration guide): clearMocks is on by default (the config already sets mockReset: true); unawaited .resolves/.rejects now fail (both uses in src/index.test.ts are awaited); no vi.mock inside functions, no sequential, no removed entry points, no coverage include/exclude, no global assignments in jsdom. The UI now requires token authentication, which only changes how pnpm coverage opens.
• jsdom 30: requires Node ^22.22.2 || ^24.15.0 || >=26.0.0. 30.1.0 also fixes the ordering of mutation observer notifications; tests pass unchanged.
• Node.js 26: removals (_stream_* modules, writeHeader(), --experimental-transform-types) are not used here. Node 26 is still "Current" and is scheduled to enter LTS in October 2026.

Test plan

• pnpm typecheck passes on Node 26.10.0
• pnpm build passes on Node 26.10.0
• pnpm test-ci passes (33 tests, type tests included)
• Suite is stable under describe.shuffle (5 runs with random seeds)
• CI test job passes

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