Add SMTP configuration to General Settings

[TimeBye]

Summary

• Add SMTP configuration to General Settings, including enablement, authentication, encryption, timeout, and sender details.
• Add test-email support using the current form values without persisting them.
• Support SMTP configuration through the smtp section in KITE_CONFIG_FILE.
• Restrict Settings access to administrators and localize the access-denied page.
• Add SMTP UI, backend, unit, E2E, and documentation coverage.

Why

Administrators need a built-in way to configure and validate SMTP delivery for future email-based features. SMTP configuration should also support configuration-file management for GitOps-style deployments.

Related issue

Closes #699

Validation

go test ./internal ./pkg/settings ./pkg/auth
npm test -- src/components/settings/general-management.test.tsx
npm run type-check
npm run lint
npx prettier --check …
git diff --check

Checklist

• I reviewed this PR myself before requesting review.
• I understand the changes, including AI-generated parts (if any).
• For new features, a feature request issue is linked.
• I cleaned up AI noise (unnecessary comments, dead code, and unrelated changes).
• This PR is reasonably scoped (or split into smaller PRs).