test(e2e): publish shared local Playwright suite

[ThisIs-Developer]

What changed

• publishes the root Playwright configuration, 109-test Chromium suite, fixtures, helpers, and locked npm dependencies
• adds a cross-platform local static server and static validation for JavaScript syntax, JSON, HTML references/IDs, and desktop-resource synchronization
• documents clean local setup and contributor-run commands in tests/README.md and wiki/Contributing.md
• removes private machine paths and unconditional screenshots from the shared tests
• fixes readiness/reload races and clipboard result handling exposed by the suite
• updates DOMPurify and js-yaml, uses valid integrity-checked CDN assets, restores local WebKit compatibility, and synchronizes the Neutralino offline resources
• keeps .nvmrc to pin Node.js 22 for reproducible local contributor installs
• adds vercel.json to preserve the existing root-served static deployment after introducing the root package.json

Local-only testing policy

This PR intentionally does not add a GitHub Actions workflow for Playwright. Contributors run the relevant checks locally before requesting review or merging. Playwright reports and failure artifacts remain local and ignored by Git.

Why

The contributing guide advertised root npm run build and npm run test:e2e commands, but the root package, Playwright configuration, and tests were excluded from version control. A fresh clone therefore could not run the maintainer's local workflow. This publishes that workflow for contributors without adding remote test execution.

Fixes #240.

Local validation

• npm ci
• npm audit --audit-level=high — 0 vulnerabilities
• npm run build — passed
• npm test — 109/109 Chromium tests passed
• npm run test:e2e:cross-browser — 9/9 smoke tests passed across Chromium, Firefox, and WebKit
• final local-only configuration check: npm run build and npm run test:e2e:smoke — 3/3 passed
• reset/reload stability check — 5/5 repeated runs passed
• git diff --check — passed

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Pull request overview

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Suppressed comments (2)

tests/e2e/smoke.spec.js:11

• The smoke test spawns the static build check via execFileSync('node', ...). Relying on a node binary in PATH can fail in constrained CI/sandbox environments; using process.execPath ensures the same Node runtime running the tests is used.

test('static build smoke check passes', () => {
  const output = execFileSync('node', ['tests/helpers/static-build-check.mjs'], {
    cwd: process.cwd(),
    encoding: 'utf8'
  });

tests/helpers/static-server.mjs:38

• resolveRequestPath() calls decodeURIComponent() on the URL pathname; malformed percent-encoding (e.g., "/%E0%A4%A") will throw and crash the static server process. Treat decode failures as a forbidden path (return null) so the server responds 403/404 instead of exiting.