Fix core tool audit follow-ups - #299

Summary

• add explicit JWT verifier states for unsupported asymmetric algorithms and alg: none instead of reporting them as invalid signatures
• support normalized 5/6-field crontab editing, preserve seconds fields, and add field-range validation
• complete audit cleanup: JSON-safe CSV cells, shared JWT base64url helpers, and undefined invalid JSON tree paths

Acceptance

• RS256/ES256-style tokens now show unsupported algorithm guidance rather than invalid signature
• alg: none shows an unsigned warning
• HS256/HS384/HS512 verification still reports valid/invalid based on the supplied secret
• * * * * * maps to the correct cron fields
• 6-field crons keep the seconds segment when editing
• invalid cron ranges return field-specific errors
• JSON-to-CSV preserves arrays/objects as JSON cells instead of lossy String() output

Validation

• npm test -- --run tests/unit/crontab-generator-logic.test.ts tests/unit/jwt-verifier-claims.test.ts tests/component/jwt-verifier-page.test.tsx tests/unit/json-formatter-tree-logic.test.ts tests/unit/csv-json-task-logic.test.ts tests/component/all-tools-discovery.test.tsx
• npm test -- --run tests/guards/jwt-security-semantics-guard.test.ts tests/guards/locale-routing-guard.test.ts tests/guards/localized-copy-guard.test.ts tests/guards/csv-json-converter-performance-guard.test.ts tests/component/json-formatter-actions.test.tsx tests/component/json-formatter-tree-node.test.tsx
• npm run check:types
• npm run lint
• npm run validate
• npm run build:app
• npm run test:e2e:smoke
• npm run test:e2e:pwa
• npm run build:post
• git diff --check

Closes #292
Closes #295
Closes #296