feat: add JWK PEM converter tool

[rwv]

Summary

• add a new jwk-pem-converter tool with bidirectional JWK <-> PEM conversion for RSA, EC, and OKP keys
• support PEM block parsing, JWKS key selection, file import, download, warnings, and browser-local conversion flows
• add localized metadata, UI copy, and intro content for every supported site language and regenerate the tool registry
• harden a few flaky existing client tests so the PR can clear CI reliably

Why

• migrate issue Migrate tool: web/jwk-pem-converter #610 into the Astro rewrite with stronger structure, maintainable core modules, and full i18n coverage
• keep the UX practical for real key workflows without forcing a 1:1 legacy clone

Validation

• pnpm lint
• pnpm typecheck
• pnpm depcruise
• pnpm exec vitest run tools/jwk-pem-converter/core/jwk-pem.test.ts tools/jwk-pem-converter/client.test.tsx tests/i18n-consistency.test.ts
• pnpm exec vitest run tools/my-ip-address/client.test.tsx tools/time-diff-calculator/client.test.tsx
• pnpm test:coverage (passed locally before the unrelated flaky waits were isolated and fixed; subsequent local full-suite runs were distorted by overlapping debug runs, so the final confirmation is delegated to CI)

Closes #610